1
general::
Enter into an activity or situation; also, intervene. For example, The business was doing poorly until Stan stepped in, or They are going to make a mess of the mailing unless someone steps in and shows them what to do. [Late 1400s] Also see STEP INTO.
American Heritage Idioms
2
general::
Phrase(s): step in something
Go to step into something., Phrase(s): step in(to some place)
to walk into a place. • Tiffany stepped into the room and said hello to everyone. • She stepped in to say hello., Phrase(s): step in(to the breach)
Fig. [for someone] to assume a position or take on a responsibility when there is a need or an opportunity to do so. • The person who was supposed to help didn’t show up, so I stepped into the breach. • The manager stepped into the breach when Jane got sick.
McGrawhill's American Idioms And Phrasal Verbs