A shelf bra is also known as a platform bra, quarter-cup bra or cupless bra. It has underwires and supports from underneath, but has no cups or very short cups to expose your breasts.
Obviously this bra is mainly just for fun. If you want a bit of lift or are smaller and want them pushed up to appear larger then this could be for you. You can also pair them with tassels if you are wanting to wear them but don’t like going braless.
Note: The term ‘shelf bra’ is also used to describe built-in support in clothing, this is quite different to the type of bra pictured, make sure you know which one is being talked about.