Plain English Meaning
Being told that another aircraft is close enough to be a concern, so you know to look for it and decide what to do.
Full Definition
A service or system function that warns a pilot of nearby aircraft that may pose a collision risk, providing awareness of conflicting traffic so the pilot can look for it and take action if needed. Traffic alerting can come from air traffic control radio calls, onboard traffic display systems (such as TCAS or TIS), or visual scanning aids, and is distinct from a collision avoidance system that issues a specific resolution maneuver.