You select the champions that you want to be able to roll. If they are semi-transparent it means that they can't be rolled from the home page.

Filter all deselects all the champions, useful if you only want a few champions to be able to be selected.

Clearbutton makes all the champions active again, so you can roll all of them.

Click the randomize button to get a random champion from the ones that are available.
Available champions are based on the selected ones in filter, and the roles selected in the home page.

all the champions in their roles that you pick on home page