I don't really understand how to use that. Can you please further explain?

I received my PI BOY Recalbox yesterday. It came pre-installed with roms. When I connect it to my PC via wifi, I can see all of the roms listed. I have tried adding some of my own roms but every time I do, it says there is not enough space. How Would I be able to add my own roms? I have tried deleting some of the pre-installed roms to make room for my rom but then I was unable to put the original roms back on the card. Once again it said not enough space. Please Help!