The loop functionality is best explained with an example.
Let's assume you wanted to ask a series of questions regarding all household members.
We first ask how many people live in your household. Assume the answer is 5 people.
The loop or step-through iterations will be 5.
So we ask questions such as age, gender, favourite movie, favourite ice cream flavour, etc. for each household member from person 1 through to person 5.
Check out the information on the Sawtooth website regarding loops. You can also watch the video demo. I'm sure you will find it quite useful.