The += assignment operator?

Hey guys I have a quick question. Could anyone explain to me what the += does and how it works? I have seen it in different examples that use arrays but I still don't get what it's supposed to do.

Here is an example I saw in my book:

Code :

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a[5] = 10;
i = 5;
a[i++] += 2;

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These two are functionally equivalent:

a += expr

a = a + (expr)

Same goes for all the other blah-equals operators (-=, *=, ...etc).

Thank you very much!