defensive replacement; late-inning defensive substitution
Baseball term for substituting fielders to protect a lead in the late innings.
In the ninth inning, they put a reserve player in right field as a defensive replacement.
They used a veteran at shortstop as a defensive substitution.
代打 is a pinch hitter (offensive substitution), while 守備固め is a defensive substitution.
Compound of 守備 (defense) and 固め (hardening, strengthening), literally 'defense hardening'.