GoLang, what is contents in parenthesis before MethodName?

Question

func (t *T) MethodName(argType T1, replyType *T2) error

what is contents in parenthesis before MethodName? I mean this (t *T)

This comes from

Answer 1

The Go Programming Language Specification

Method declarations

A method is a function with a receiver. A method declaration binds an identifier, the method name, to a method, and associates the method with the receiver's base type.

Given type Point, the declarations

func (p *Point) Length() float64 {
    return math.Sqrt(p.x * p.x + p.y * p.y)
}

func (p *Point) Scale(factor float64) {
    p.x *= factor
    p.y *= factor
}

bind the methods Length and Scale, with receiver type *Point, to the base type.

It's the method receiver.