go - Function declaration syntax: things in parenthesis before function name

Question

I'm sorry I couldn't be more specific in the question title, but I was reading some Go code and I encountered function declarations of this form:

func (h handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    ...
}

from https://github.com/mattermost/platform/blob/master/api/context.go

func (s *GracefulServer) BlockingClose() bool {
    ...
}

from https://github.com/braintree/manners/blob/master/server.go

What does the (h handler) and the (s *GracefulServer) between parenthesis mean? What does the entire function declaration mean, taking into account the meaning of the things between parenthesis?

Edit

This is not a duplicate of Whats the difference of functions and methods in Go? : this question came to me because I didn't know what the things in parenthesis before the function name were, not because I wondered what was the difference between functions and methods... if I knew that this declaration was a method I wouldn't have had this question in the first place. If someone has the same doubt as me one day, I don't believe she will go searching for "golang methods" because she doesn't know that this is the case. It would be like wondering what the letter "sigma" means before a mathematical expression (not knowing it means summation) and someone says it's a duplicate of what's the difference between summation and some other thing.

Also, the short answer to this question ("it's a receiver") is no answer to "what's the difference between functions and methods".

Answer 1

Waitting for answers