step -- specifies the increment step in a for-loop
for a=1 to 10 step 3
Specify, by which amount the loop-variable of a for-loop will be incremented at each step.
The step (as well as the lower and upper bound) are computed anew in each step; this is not common, but possible, as the example below demonstrates.
for x=1 to 1000 step y
print x," ",y," "
This program computes the fibonacci numbers between 1 and 1000.