High-grading is a kind of logging where loggers take only the biggest and most financially valuable trees. It yields a lot of money upfront, but in the long run it's bad for the forest. It removes the trees that were best-adapted to your woods and leaves you with trees that are smaller and slower growing.
Unfortunately high-grading is pretty common because of the short-term cash benefits. It sounds like in your case the previous landowners "cashed out" and sold off the high-value timber to get as much money as possible before selling the property.
I'd suggest hiring a professional forester to come out and visit your woodlot. He or she would be able to advise you on next steps. There are ways to restore your woods after a high grade, but you may have to be willing to invest some money into bringing your woodlot back to a healthy condition.