Selectors are basically clever getters. They cache their return values.
They only update their return values if an attribute they depend on changes.
The word is taken from redux, but I guess you could think of it as - a selector selects attributes and combines them in some way.
Mobx uses the word compute, as in you re-compute the value if an attribute changes.
In most cases a selector decorator isn’t required (eg in the toy example above it’s not worth caching the value for get_count). It’s most useful for an expensive getter.