Legacy GM Basic movement, problem with operator (+=) [SOLVED]

[TheXGuy]

Hello, I was following a tutorial and trying to understand every step of it but I'm stuck on this. I don't know why he uses this line:

x += hsp;​

Instead of

x = hsp;​

Actually I don't really understand what the operator += or -= means.

The full code is shown in this picture.

http://i.imgur.com/FmWcOQJ.jpg

 

[RealsLife]

Because in the code if you push vk_left or whatever
let's say hsp = 3;

If you use this x = hsp; for example the position of x in the object becomes the value 3 and we don't want that instead we want the current position of x = x + hsp(or in this case 3), because x += hsp is actually a shorter version of x = x + hsp.

 

[Bingdom]

+= is adding the value
-= is subtracting the value
= Is setting the value

 

[TheXGuy]

Thank you both, now I can see how it works.