It a menu system for the DSTT that someone named Yasu wrote. You can use it in place of the OEM menu (TTMenu) that you would normally run on the DSTT.
Advantages of YSMenu over TTMenu: it sorts the roms, it shows folders allowing you to organize your roms into separate screens, it attempts to patch the browser and GBA games without having to use a separate loader program, it runs emulated programs directly in the emulator in one step.
Disagvantages of YSMenu: it's not very pretty (IMO), it doesn't support the touch screen very well (you have to use the buttons to do most of the operations), it doesn't support download play.
It is possible to install both YSMenu and TTMenu on the same card, and you can switch back and forth between the two as needed.
-Bri