What is the meaning of Drop-in?

Provided for short-term use.

Fit to substitute for some element in a complex system without changes to the existing infrastructure.

Allowing people to drop in; that is, to visit casually, without an appointment.

Of the pitch: prepared away from the ground or venue, and dropped into place only when a match is to be played.

One who casually drops in (visits unannounced or without appointment).

An informal event that does not require booking in advance.

A place that can be visited casually, without an appointment.

The act of dropping in; that is, taking a wave that another surfer is already riding.

A pitch that is prepared away from the ground or venue, and dropped into place only when a match is to be played.

Source: wiktionary.org