MTF charts are graphs of lens resolution that are meant to give you an idea of how a given lens performs at different points across the frame. (Most lenses are sharpest in the centre and get a bit softer as you go out, and these charts illustrate that characteristic.) They’re easy to read once you understand them, but unfortunately they’re usually explained very badly.
Mike Johnston has put together the first sane and easy-to-understand breakdown I’ve ever seen. Check it out at The Online Photographer.