It means ServeHTTP is not a standalone function. The parenthesis before the function name is the Go way of defining the object on which these functions will operate. So, essentially ServeHTTP is a method of type handler and can be invoked using any object, say h, of type handler.

h.ServeHTTP(w, r)

They are also called receivers. There are two ways of defining them. If you want to modify the receiver use a pointer like:

func (s *MyStruct) pointerMethod() { } // method on pointer

If you dont need to modify the receiver you can define the receiver as a value like:

func (s MyStruct)  valueMethod()   { } // method on value

This example from Go playground demonstrates the concept.

package main

import "fmt"

type Mutatable struct {
    a int
    b int
}

func (m Mutatable) StayTheSame() {
    m.a = 5
    m.b = 7
}

func (m *Mutatable) Mutate() {
    m.a = 5
    m.b = 7
}

func main() {
    m := &Mutatable{0, 0}
    fmt.Println(m)
    m.StayTheSame()
    fmt.Println(m)
    m.Mutate()
    fmt.Println(m)

The output of the above program is :

&{0 0}
&{0 0}
&{5 7}