expression, ichidan verb
get on base
Baseball term for a batter reaching base safely.
彼は四球で塁に出た。
He got on base with a walk.
ヒットを打って塁に出る。
Get on base with a hit.
出塁 is a noun meaning 'reaching base', while 塁に出る is the verbal phrase 'to get on base'.
Compound of 塁 (base) + に (particle) + 出る (to go out).