In radar-controlled airspace, ATC provides traffic advisories to alert you to other aircraft in your vicinity. There are several types of traffic information:

Traffic Advisory: ATC has observed a target on radar at a specific location and altitude and believes it may be a factor in your flight. For example:

F-LFPG, TRAFFIC, TEN O'CLOCK, FIVE MILES, NORTHEAST BOUND, CLIMBING THROUGH FLIGHT LEVEL TWO SEVEN ZERO, ESTIMATED SEPARATION FIVE HUNDRED FEET VERTICAL.

You should locate this traffic visually if possible and report:

TRAFFIC IN SIGHT or NEGATIVE CONTACT (if you cannot see it).