In this folder you could have the genome index for bowtie. Of course you can use your own index if you have. Otherwise use the commands below to download and unzip the genome index. Examples are provided here for human, mouse, and rat.
wget https://genome-idx.s3.amazonaws.com/bt/GRCh38_noalt_as.zip # for downloading
unzip GRCh38_noalt_as.zip # for unzipping
wget https://genome-idx.s3.amazonaws.com/bt/mm10.zip # for downloading
unzip mm10.zip # for unzipping
wget https://genome-idx.s3.amazonaws.com/bt/Rnor_6.0.zip # for downloading
unzip Rnor_6.0.zip # for unzipping