Transonic means breaking the sound barrier by travelling faster than the speed of sound (1,236 kph or 768 mph).
The speed of flight is divided into four groups based on speed.
Transonic means going from subsonic to supersonic. Subsonic ends and supersonic begins at mach 1, so transonic goes from mach 0.8 to 1.2. When you are going through transonic speeds, drag increases and it is really hard to go any faster. This is the sound barrier.