TCAS is the "traffic Alert & Collision Avoidance System", it uses the aircraft transponder system to work out where other aircraft are in the sky and if they get too close it shouts at the pilots to point their aircraft in a direction (climb or descent) to prevent the knocking together of expensive metal machines.
The transponder is a small device that sends out a code that is assigned to a flight by ATC. The ATC radar recognizes this code and matches up flight details with the radar blip to provide the controllers with the info they need to stop the aircraft getting too close in the first place.
When you enter the runway you select the transponder to TA/RA setting. This is "Traffic Advisory / Resolution Advisory" which switches TCAS to the setting where it will shout to resolve any conflicts.
Other setting are STBY, XPDR (ATC transponder function only) and TA Only.
See
http://www.caasd.org/proj/tcas/
for more details on this clever system.
