A Direct Station Selection (DSS) key is used on a phone or sidecar to allow monitoring and/or direct dialing of an extension. These keys can also often be programmed as one touch dials for external numbers as well. Sometimes they are referred to as Busy Lamp Field (BLF) keys.
There are a number of specific DSS supported phones such as:
Yealink - many models support both integrated DSS functions (on the phone) as well as external modules (EXPXX) - a comparison of expansion modules can be found here.
Cisco - the SPA500DS can enable 30 DSS buttons on any SPA500 series phone