A expression is something of the form , which means "take in an input , and return \. For example, takes in a number and returns one more than it\. A expression can have multiple inputs; e\.g\. takes in two numbers and returns their sum\. We can also write this as ; "" is simply a shorthand for "\."
I think it's confusing to introduce multi-argument functions before talking about currying. This makes it seem as though multi-argument functions are an intrinsic part of the lambda calculus, rather than just functions that return other functions.