There are two words that can summarize what the double bars represent:

1.Magnitude

2.Norm

Its just asking you to find the length of the vector; and it can be done by applying this definition:

----- [Two Dimensions]

----- [Three Dimensions]

----- [ Dimensions]

You can prove the two and three dimensional space by applying the Pythagorean Theorem. This then, can be generalized for a vector that has components.

Does this help?

--Chris