Definition: the ruffled frill that forms around the base of a macaron shell as it bakes — the sign of a well-made macaron.
The “feet” (les pieds) are that little ruffled skirt at the bottom of each shell. They happen because you rest the piped shells until a skin forms on top; in the oven the batter can’t rise through that skin, so it pushesout at the base instead, and the frill you get is the feet. No feet usually means the batter was undermixed, the shells weren’t rested long enough, or there was too much moisture about.
In practice: rest until the tops are dry to a light touch — that skin is what makes the feet.
Related:macaronage,French meringue.