\gets stands for assignment, like = in most programming languages.
For example, a = 5 (in say Python), and a \gets 5 both represent assigning a value of 5 to a.
the usage of \gets or := in assignment sentences is to avoid confusion with comparing equality as in =. Namely:
what does \alpha=5 mean? does it mean compare \alpha with 5 (perhaps as a condition in an IF statement)?
This is why people often write assignment as \alpha\gets 5 or \alpha:=5.