Trailbraking is a term used to describe carrying the braking action of a vehicle past the turn entry.
In motorcycle parlance, the rider applies either or both brakes at varying braking pressure between the front/rear. The pressure to use is determined by the rider's entry speed, lean angle, and other factors. The use of trailbraking allows the rider to adjust speed all the way through a turn, and can allow the rider to set the motorcycle's cornering speed at the apex of the turn -- the slowest point of a corner -- or even past the apex, if necessary. A motorcycle (or motorbike) is a two-wheeled vehicle powered by an engine. ...
This technique is commonly used when racing, but can enhance control and add more evasive options for advanced street riders.
References
Ienatsch, Nick (2003). Sport Riding Techniques: How to Develop Real World Skills for Speed, Safety and Confidence on the Street and Track. David Bull. ISBN 1-893618-07-2.
I don't use trailbraking on the street unless I've done something thoroughly stupid, and it is a last ditch effort to stay upright.
Trailbraking with the front brake is something you must practice.
It is amazing how much front brake you can use leaned over in a corner, but you'll never know how far you can be leaned over, or how much brake you can use unless you practice until becomes instinctive and reactive.