The loop block is used to loop the same script several times.
It has two inputs, Start and Stop. The loop block starts looping from the Start input, and stops looping once it reaches the Stop value. If the Start input is greater than the Stop input, it will count down instead of counting up.
While the loop block is executing, it will execute the yellow Do wire at every loop. Connect this wire to the scripts you would like to loop.
The counter output will output the current loop number, AKA the current iteration.