An Asset is a single translatable item
You may be familiar with terms such as "strings" or "messages", and depending on your development platform these may mean slightly different things. Think of Loco assets as containers for one or more translations of the same thing. Usually that thing will be a short phrase used in a specific place in a software product, e.g. a menu item, or page title.
Each asset is unique within a Loco project and can be translated into as many languages as you need. It is these individual translations that Loco's storage quotas are based on. For example, 100 assets across 5 languages uses 500 translations of your quota.
Translations can contain any number of words or characters, but in software development they tend be short phrases or sometimes blocks of code. If you're interested in translating long documents (like Word or PDF files) then Loco is probably not the tool for you.