A loop allows you to repeat a set of blocks an indeterminate amount of times. The canonical example is the challenge of collecting information about the end-user's children; you don't know how many children they may have, so you often should you ask for a child's name? A loop solves this problem.
A loop is comprised of members. In the example above, each child is a different member of the loop. You can pull out of the loop the data of a specific member.
In order to use data from all loop members, you must first instruct the app on how to organize that data. For example, if you want all the names of children in the loop, how should the app organize those names? As a bulleted list? As a comma delimited list?