Could someone explain what this means?!

You would have to first find out the relative molecular mass of the compound- like for methane, this would be 16 (as carbon has an Ar of 12 and hydrogen has an Ar of 1, and there are 4 of them).
Then, you would divide the mass by the Mr to find out how many miles you have- for your methane example, that would be 4/16, which is 0.25mol.

You can use the equation " Mass= Mr x Mole" In your questions you're given two of those things and need to find the third. Rearrange the equation to get the value that you need.