Resolution of gel electrophoresis
Resolution size of the gel simply means the distinction that can be made between molecules very close in size.

If you were looking to distinguish between a 100bp fragment and a 200bp fragment, then agarose will give you that level of differentiation. 

But if you were looking to distinguish between a 190bp fragment and a 200bp fragment then you would likely want to us polyacrylamide. The PA gel has a higher resolution because you can distinguish between smaller differences on molecule length. 

Think of it in terms of a simplified idea of a microscope, at 100X, you can probably distinguish between cells, maybe make out a well stained nucleus. where as at 1000X you can start to discern other organelles. You are seeing a finer degree of detail at the higher magnification. There are many factors that go into the resolving power of a microscope and the example based on magnification is simplistic and doesn't really address optical resolution, but it is an illustrative example of resolution in a gel. You can think of agarose as the 100X and PA as the 1000x, if that helps.