let expression presented without a proper description

[ghost]

• My programming background is ...
  I am a javascript and python web developer

• I want to learn Elm because ...
  I'd like to twirl my old, oop and procedural mind. Elm seems the more practical way to introduce myself to fp. Learning Elm since 3 days in my spare time, so I am pretty fresh :)

• I was confused by ...
  The let expression is first encountered here: https://guide.elm-lang.org/effects/time.html
  Maybe a proper description would be nice, or at least a link to https://elm-lang.org/docs/syntax#let-expressions ?

I don't know if that is a common construct for fp languages, but it was pretty new to me.