The config file

The config file is long and "somewhat" organised, but not optimal not definitely not self-explanatory.
Some of these are also explained in game_readme.txt.
If you don't know how to open .ini files, any text editor should work, such as Notepad.
Keep in mind that 0 means OFF and 1 means ON. "n" or "x" means any number.
Here is an explanation of every single item in the config file:

[Global]

Skin = foldername Set your skin. Change the value "foldername" to the foldername of your skin found in /GameData/Skins/
Widescreen = 0/1/2 0 makes the game 4:3, 1 makes the game 16:9 and 2 makes the game 16:10.
WindowWidth = x Width of the game in pixels, if 0 or blank, it will use your monitor resolution.
WindowHeight = x Height of the game in pixels, if 0 or blank, it will use your monitor resolution.
Fullscreen = 0/1 Fullscreen mode. Not borderless.
VideoFlush = 0/1 Finish all OpenGL commands before swapping
VSync = 0/1 Turns VSync on, limiting FPS to 60 but with significant input delay.
ControllerNumber = x Change x to the controller you want to use. Usually you only need to set it to 1.
VSRGEnabled = 0/1 Enables "VSRG" mode (aka falling notes). VSRG is short for vertically scrolling rhythm game.
dotcurEnabled = 0/1 Enable dotCur mode. Leftovers from the time the game was titled dotCur and had a Cytus mode.
AudioCompensation = 0/1 Try to automatically sync charts.
InterpolateTime = 0/1 When using a song with streamed music rather than samples, when enabled, tries to interpolate between audio frames rather than staying 100% on sync with the music.
UseAudioCompensationKeysounds = 0/1 Applies audio latency compensation to keysounded charts. Only when AudioCompensation is enabled!
UseAudioCompensationNonKeysounded = 0/1 Applies audio latency compensation to non keysounded charts. Only when AudioCompensation is enabled!
AlwaysUseRaindropMechanics = 0/1 Force standardized scoring for all charts of any format. This is implemented because o2jam charts will get a unique scoring method akin to o2jam. Enabling this forces it to use the regular scoring method.
AlwaysUseRidiculousTiming = 0/1 Super strict timing, akin to GAMBOL mode.
ErrorTolerance = x Sets how much InterpolateTime can desync with. Measured in milliseconds. Offset7K = x Offsets the NOTES for osu!mania charts.
DisableHitsounds = 0/1 Disables sounds, only works for osu!mania charts.
Preload = 0/1 0 makes the game load your directories after starting the game. This means you can play songs whilst waiting for others to load. Setting it to 1 means the game will load all songs before starting.
DefaultJudgeRank = 1~4 Set timing, 1 is the easiest, 4 is the strictest.
DisableBGA = 0/1 Disables BGA for BMS.
KeyProfilen = profile Set which profile nkeys mode uses.
Keys = Left unused.
OffsetKeysounded = x Set offset for keysounds in seconds (Decimals allowed).
OffsetNonKeysounded = x Set offset for non-keysounded charts in seconds (Decimals allowed).
DisableKeysounds = 0/1 Whether you want your BMS files to use keysounds as feedback, otherwise rendered as the BGM track.
NoFileGrouping = 0/1 Whether chart files can be grouped. (Requires database regen)
SeparateBySubtitle = 0/1 Whether the charts are separated when different subtitles are detected. (Requires database regen)
JudgeOffsetMS = x Set judge offset in milliseconds.

[SystemKeys]

key = function Set which key shall trigger which function.

[Keys7K]

key = n Set which key (or keycode) shall trigger which in-game key during play.

[ProfilenK]

Keys = x Set which keys apply for which profile. E.g. Keys = 1,2,3 for [Profile3K]

[Audio]

UseWasapi = 0/1 Enables WASAPI. This greatly reduces sound latency. Only works if your sound card is set to 44100 Hz mode.
UseThreadedDecoder = 0/1 When decoding audio, use a separate thread instead of the main thread.
UseHighLatency = 0/1 Uses high latency values provided by portaudio rather than the low latency values. Mostly useful with ALSA (linux)
RequestedLatency = x The nominal latency you want to ask from the system (in seconds). May differ from real latency.
WasapiUseSharedMode = 0/1 Exclusive mode means the game will hoard your sound card. No other applications can use your sound card, making them unable to give off sound. Enabling this will turn it off exclusive mode, allowing other applications to use the sound card too.

[SongDirectories]

title = directory Add directories raindrop looks for songs in. Both values "title" and "directory" can be changes. "title" is the title of the folder displayed on the song wheel, whilst "directory" is where the folder is located on your PC.

[Debug]

In this section you can enable and disable debug messages. Useless for most users.

[NPS]

This section contains NPS graph generation settings.
IntervalTime = x The time to use for each point of the graph.
PeakMargin = x How much from the peak NPS should you have of extra space vertically (factor)
GraphHeight = x Height of the NPS graph.
GraphYOffs = x Offset in Y coordinates of the graph.
GraphXOffs = x Offset in X coordinates of the graph.
Width = x Width of the NPS graph.
TextXOffs = x Offset in X coordinates of the title.
TextYOffs = x Offset in Y coordinates of the title.
YAxisMarkers = x How many horizontal lines to draw on the graph with the NPS value on them.