This is sort of an arbitrary term to define, but basically this describes something that has a stem and base or legs attached to a bowl, cup, etc.
As always, it’s best to show it in pictures. First we have a typical “footed bowl,” with three or more legs. Sometimes these can look like actual feet (usually of animals, like a lion), other times they’re just plain legs found at the bottom of the bowl.
The second type is also often referred to as footed, or even “stemmed.” This is typically seen with glasses that have a long stem, then a round base at the bottom.
It’s as easy as that!