Answer:
If your talking about multiplication, then when you double something, your multiplying it by two, for example, "double 16 and then add 20." In this statement, you multiply 16 by two which will give you 32. Then you add 20, giving you 52.
If this isn't what your talking about, let me know or give me an example and I'll tell you what it means.
In short, its multiplying something by 2.