The "Share" folder in Recalbox is the default folder you can access on the network by connecting to your recalbox using a file manager, it is also the folder named "Recalbox" on an external storage when it's used.
This folder let's you manage most files and subfolders intended for a average user, it contains the following folders:
The folowing folders are related to the games, and should be backup frequently to prevent any loss, it is also wise to back them up before updating just in case.
The bios folder has to contain the BIOSes files used by Recalbox consoles emulators, please note that some emulator do not require any BIOS to work out of the box, while others will.
The saves folder will fill itself as you play your games, it will contain subfolders (up to one for each system), with each of them having savestates files as well as original saves for each game involved.
The roms folder contains multiple subfolders, one for each supported system, each of them contains a readme.txt file describing the supported game format for each system, the games are meant to be inside those, you also have gamelist.xml files, and potential media subfolders. The gamelist.xml contains informations for each game, in a way EmulationStation can understand, and the media subfolders contains images or videos to be displayed along the games in the game select screen.
The music and themes folder can be used to alter the look of EmulationStation.
By putting musics in a ogg vorbis format inside the music folder, EmulationStation will use those musics instead of the default ones.
You can also put custom themes inside EmulationStation and enable them using the system menu.