Originally Posted by dkul

"Traffic, Traffic!!!" is the TCAS call out for a TA:traffic advisory.
To let the pilots know that a plane is getting close, in visual conditions usually not a problem.
The next level is an RA:resolution advisory. An RA can come in various call-out: "Descend", "Climb", "Monitor Vertical Speed", to name a few. This means a collision will most probably happen if the action as directed is not taken.