expression, godan verb (-ru, irregular)
have legs; be mobile; be a good runner
Used literally for having legs, or figuratively for being able to move around well or run fast. Often appears in contexts describing physical ability or mobility.
彼は足があるから、遠くまで歩ける。
He has strong legs, so he can walk long distances.
あの選手は本当に足があるね。
That athlete really is a good runner, isn't he?
足が速い specifically means 'fast-footed' or 'quick runner', while 足がある can imply general mobility or running ability without emphasizing speed.
Compound of 足 (foot, leg) + が (subject marker) + ある (to exist, to have). Literally 'legs exist', used idiomatically to mean having the ability to walk or run.