Definition of "step in"

step in

verb
third-person singular simple present steps in, present participle stepping in, simple past and past participle stepped in
1. (intransitive) To act as a replacement or substitute.
  Quotations
  McCoist unexpectedly ushered back a defender of his own with Kirk Broadfoot taking over from Steven Whittaker. There was, of course, another change, Kyle Bartley stepping in at centre-half to replace suspended Dorin Goian.
  2011 December 29, Keith Jackson, “SPL: Celtic 1 Rangers 0”, in Daily Record‎
2. (intransitive) To get involved; to act