Potty chairs are proportionately small chairs or enclosures with an opening for seating very young children to "go potty" (a.k.a. poop, defacate, crap, make kaka, etc.). It is a variant of the close stool

Close stool

A close stool, used from at least the sixteenth century until the introduction of indoor plumbing, was an enclosed cabinet or box at sitting height with an opening in the top, which might be disguised by a folding outer lid. The close stool contained a pewter or earthenware chamberpot to receive...

 which was used by adults before the widespread adoption of water flushed toilets. There are a variety of designs, some placed directly over the toilet so the egested fecal material (poop) drops directly into the toilet bowl thereby eliminating manual removal and disposal of the said poop from a receptacle beneath the hole which is often a bag or receptacle similar to a chamber pot

Chamber pot

A chamber pot is a bowl-shaped container with a handle, and often a lid, kept in the bedroom under a bed or in the cabinet of a nightstand and...

. If the child stays in the correct position, of course. Potty chairs are used during potty training a.k.a. toilet training.