What is a "receiver"?

Not sure I can point you to a tutorial about it or anything specificly about this. There is a short paragraph about it in the official documentation:
https://kotlinlang.org/docs/reference/lambdas.html#function-types

Function types can optionally have an additional receiver type, which is specified before a dot in the notation: the type A.(B) -> C represents functions that can be called on a receiver object of A with a parameter of B and return a value of C . Function literals with receiver are often used along with these types.

As well as a part at the end of that page:
https://kotlinlang.org/docs/reference/lambdas.html#function-literals-with-receiver

This SO answer could also be helpful

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