Assessment Of My Dad's Collection

I know nothing about stamp collecting, so pardon my ignorance. My dad passed away a couple of years ago, and my mom has been slowly rummaging through boxes of things he had stored in the basement. Recently she discovered what appears to be a collection of uncancelled stamps, in blocks of four, primarily from the USA. We don't know what to do with this "collection," so any suggestions will be welcomed. We are not especially interested in parting with this group of literally thousands of stamps, but we are curious about the significance of my dad's clandestine hobby. Thanks for your comments or advice .......... Craig