Bento just refers to the bunch of extra bones in newer avatars that move in more complex ways. By default the Maitreya body comes with bento hands which you attach to the ends of your arms (you could also use the older non-bento hands too). Normally the nails built into the hands are displayed unless you turn them off and they have three different size options. Bento nails are basically little objects that get attached to each finger and they are usually sold in one length as a set though there may be designers who sell different size nails together. I'd go to some nail shops and demo some nails to see what sizes, shapes, colors, patterns, extra bits like rings, etc you like. For Matreiya hands you need nails rigged to fit right otherwise your nails will be floating around away from your fingers or embedded in the tips. Since those nails are stuck on the ends of your fingers that's why you need to hide the original ones.