In a generic sense, an interface is a boundary between two different systems or sub-systems.

The most commonly defined interfaces for space systems are

Mechanical Interface

How the sub-system is mounted onto another system or subsystem;

Thermal Interface

How much heat the sub-system conducts via contact surfaces, or radiates, to other subsystems;

Electrical Interface

How the sub-system is connected electrically to other sub-systems (connectors, voltages, currents, ...);

Software Interface

How the sub-system communicates with other sub-ssytems (protocols, message layouts, meaning of messages)

This list is not exhaustive, and other interfaces may be defined as needed (TM/TC interface, RF interface, ...)

The interfaces are usually defined in a Interface Requirements Document (IRD), and described in an Interface Control Document (ICD).