A screener in basketball is just as it sounds, someone that sets a screen. A screen is also known as a "pick" and involves blocking a defender's path to allow the ball handler to get by them. The screener sets stationary picks from anywhere on the offensive half of the court. Usually before the set the screen they motion to the point guard or are motioned over. The image below is a good example of what a screen looks like.
Screens can also be set to free up players without the ball. An example of when this would be beneficial is a backdoor cut. The screener frees up an offensive player without the ball who is then able to cut to the basket and receive the ball unguarded.
Once a screener actually sets the screen, they have options of where they can go.
One option is the pick and roll. Immediately after setting the pick, the screener pivots and runs through the lane towards the basket. Ideally the ball handler will see this and pass them the ball. This works best if the defense double teams the ball handler when the screen is set.
Another option is the pick and pop, or pick and fade. Instead of running to the basket, the screener "pops" out and back pedals to the three point line. A lot of times the defense will forget about the screener and that can result in open three point shot attempts.
When setting a screen a screener needs to be wary of committing a violation. When a violation is committed their team loses possession of the ball and a personal foul can be called on the screener. Such violations are known as illegal screens.
The main illegal screen that is called in basketball is the moving screen Once the screener sets their position for screening, they are no longer allowed to move. If they move into the opposing player while they are setting a screen, a foul is called.