Interface is the crossing point or technical connection between different IT systems. It is the direct communication/ connection between two (or more) networks/ applications/ systems that can be interlinked together without any intermediary.
The short video below courtesy of Pegasus; explains the interface between PMS and CRS. A one-way interface generally refers to pushing of reservations from one system to another. e.g. CRS to PMS, or Channel Manager to a PMS.
Two-way interfaces include the reservations push plus rates, availability and stay restrictions both ways between the systems.
There many interfaces available with hotel software such as point of sale (POS), door locking, back office accounting etc. The diagram at the bottom of the page shows a small sample of possible system interfaces that hotels / resorts may have in place.