A Loop refers to a single Loop of time for a given universe. The amount of time that a Loop lasts can vary, though most universes tend to have a set time when Loops start and stop. (Bleach loops are an exception as they tend to last untill the death of this universe.)
Loops are usually published in compilations of short snippets. Each snippet tends to describe a single Loop, though it may reference other Loops, or contain a transition between one Loop to another. Snippets are anywhere from a few sentences to multiple paragraphs, depending on how in-depth an author decides to be.
A Loop is not considered to be canon unless it is included or planned to be in a published work. Even then, if it contradicts previously established Loops, it may be disregarded by some.
Canon of the looping story.
A Hub loop is a universe like our world with nothing overly exciting or fictional occurring. It's considered an easy loop and often used as a vacation by loopers. To quote user Crisis on Spacebattles:
☀Hub universe - 'Ours'. Every Loop exists as fiction here and it is being used to Anchor the entire multiverse. No actual Anchor (or other loopers for that matter) needed. Keeping the Hub stable enough to run the Loops is taking up most of Yggdrasil's remaining power.