Английский словарь американских идиом - step in

{v.} 1. To go inside for a quick visit. * /It was a cold night, and when the policeman passed, we invited him to step in for a cup of coffee./ 2. To begin to take part in a continuing action or discussion, especially without being asked. * /When the dogs began to fight, John stepped in to stop it before they were hurt./ * /When Bill had done as much as he was able to on his model plane, his father stepped in to help him./