is a notation that represents the "length" of a vector

. In your expression,

is the vector whose length is being calculated.

Since "length" can be defined in a number of ways, the subscript

indicates that the length is calculated using the

Euclidean norm. So,

means that you should find the length of the vector

as defined by the Euclidean norm.

The Euclidean norm is the most intuitive definition of length. For example, if you have a vector

, its Euclidean length would be

. You can conceptualize this as the distance between the point

and the origin.