let is used to declare a constant that you cannot change later.
var is used to create a variable that you may change later.
For example, to create a constant, use
let as follows:
let num = 100.0
And to create a variable, use var like this:
var x = 200
Notice how Swift strongly advices you to create constants whenever possible by showing you warnings in Xcode. This makes sense, because if you think about it, all it tries to do is to prevent you from accidentally changing values that are not meant to be changed.
Thanks for reading. Happy Coding!