What is the meaning of Drop_in?

Used other than figuratively or idiomatically: see drop, in.

To arrive casually and unannounced, with little or no warning; also, to visit without an appointment.

To paddle into and take off on a wave another surfer is already riding.

To fall in; to cave in.

To install components, particularly car engines.

Alternative spelling of drop-in.

Source: wiktionary.org