GO using interfaces as fields

I'm going through An Introduction to Programming in Go and trying to grasp interfaces. I feel like I have an ok idea of what they are and why we would need them but I'm having trouble using them. At the end of the section they have

>Interfaces can also be used as fields:
>
type MultiShape struct {
shapes []Shape
}
>We can even turn MultiShape itself into a Shape by giving it an area method:
>
func (m *MultiShape) area() float64 {
var area float64
for _, s := range m.shapes {
area += s.area()
}
return area
}
>Now a MultiShape can contain Circles, Rectangles or even other MultiShapes.

I don't know how to use this. My understanding of this is MultiShape can have a Circle and Rectangle in it's slice

This is the example code I'm working with

package main
import ("fmt"; "math")
type Shape interface {
area() float64
}
type MultiShape struct {
shapes []Shape
}
func (m *MultiShape) area() float64 {
var area float64
for _, s := range m.shapes {
area += s.area()
}
return area
}
// ===============================================
// Rectangles
type Rectangle struct {
x1, y1, x2, y2 float64
}
func distance(x1, y1, x2, y2 float64) float64 {
a := x2 - x1
b := y2 - y1
return math.Sqrt(a*a + b*b)
}
func (r *Rectangle) area() float64 {
l := distance(r.x1, r.y1, r.x1, r.y2)
w := distance(r.x1, r.y1, r.x2, r.y1)
return l*w
}
// ===============================================
// Circles
type Circle struct {
x, y, r float64
}
func (c * Circle) area() float64 {
return math.Pi * c.r*c.r
}
// ===============================================
func totalArea(shapes ...Shape) float64 {
var area float64
for _, s := range shapes {
area += s.area()
}
return area
}
func main() {
c := Circle{0,0,5}
fmt.Println(c.area())
r := Rectangle{0, 0, 10, 10}
fmt.Println(r.area())
fmt.Println(totalArea(&r, &c))
//~ This doesn't work but this is my understanding of it
//~ m := []MultiShape{c, r}
//~ fmt.Println(totalArea(&m))
}

Can someone help me with this? I have a python background so if there is some kind of link between the two that would help.

Thanks

For example,

package main
import (
"fmt"
"math"
)
type Shape interface {
area() float64
}
type MultiShape struct {
shapes []Shape
}
func (m *MultiShape) area() float64 {
var area float64
for _, s := range m.shapes {
area += s.area()
}
return area
}
type Rectangle struct {
x1, y1, x2, y2 float64
}
func (s *Rectangle) area() float64 {
x := math.Abs(s.x2 - s.x1)
y := math.Abs(s.y2 - s.y1)
return x * y
}
func (s *Rectangle) distance() float64 {
x := s.x2 - s.x1
y := s.y2 - s.y1
return math.Sqrt(x*x + y*y)
}
type Circle struct {
x, y, r float64
}
func (s *Circle) area() float64 {
return math.Pi * s.r * s.r
}
func main() {
r1 := Rectangle{1, 1, 4, 4}
fmt.Println(r1.area())
r2 := Rectangle{2, 4, 3, 6}
fmt.Println(r2.area())
c := Circle{10, 10, 2}
fmt.Println(c.area())
m := MultiShape{[]Shape{&r1, &r2, &c}}
fmt.Println(m.area())
}

Output:

9
2
12.566370614359172
23.566370614359172