Need help understanding a problem

Hi all,
I need help understanding a problem given in the Accelerated C++ book on page 100, problem 5-5.

The problem states

Code:

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Write a function named center(const vector<string>&) that returns a picture in which all the lines of the original picture are padded out to their full width, and the padding is as evenly divided as possible between the left and the right sides of the picture.  What are the properties of pictures for which such a function is useful?  How can you tell whether a given picture has those properties?

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So this function, center, takes in a vector of strings, but the problem talks about a picture. Can somebody help me get a clear understanding of this problem? Thank you!

The picture is probably ASCII art since the representation is a string vector.

what does it mean with this part?

Code:

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...which all the lines of the original picture are padded out to their full width

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