This refers to encoding and decoding encryptions. There are many different types of encryptions - from the age old Caesar shift to RSA. In EasyCTF specifically, the problems generally involve decoding the given encryption. Many of these problems get surprisingly difficult very fast but once you understand how to do one of them, the chances are that you will start being able to get all of them. There always is a need to research as EasyCTF often brings unexpected and unknown variations to standard types of encryptions (especially RSA).