Canon EOS Rebel T5 18-55 IS II Kit Manual (en)

Depending on the scene, position the subject toward the left or right to 
create a balanced background and good perspective.
In the <A> mode, while you press the shutter button halfway to focus 
on a still subject, the focus will be locked. You can then recompose the 
shot and press the shutter button completely to take the picture. This is 
called “focus lock”. Focus lock is also possible in other Basic Zone 
modes (except <5>).
In the <A> mode, if the subject moves (distance to camera changes) 
while or after you focus, AI Servo AF will take effect to focus on the 
subject continuously. (The beeper will continue beeping softly.) As long 
as you keep aiming the AF point on the subject while pressing the 
shutter button halfway, the focusing will be continuous. When you want 
to take the picture, press the shutter button completely.
 Full Auto Techniques (Scene Intelligent Auto)
Recomposing the Shot
Shooting a Moving Subject