The previous two Yelpers certainly explained the tavern well enough but I'm going to throw in a few details that they overlooked.
First and foremost is that it's an oddity of history that George Washington came to own the land on which he fought his first battle. He fought his first battle at Fort Necessity right down the hill from where the tavern stands in 1754. As history will tell you, things didn't turn out so well for young Colonel Washington.
But the area stuck with Washington and fifteen years after his defeat at Fort Necessity he came back to buy the land consisting of 234 1/2 acres in southwestern Pennsylvania. The tract of land was called "Mount Washington" and embraced the Great Meadows. However, little did he know then that this tract would later boast a tavern whose story would live for generations.
Although Washington never lived to see his land flourish, he knew it would be an excellent site for an inn. Soon after his death the first federally funded road was laid down through this country, crossing the Mount Washington tract, thus proving his speculations correct.
Travelers stopping at the Mt. Washington Tavern were pleased to find a commodious brick and stone tavern of eleven rooms in two stories, with an attic and basement. Built around 1827 by Judge Nathaniel Ewing of Uniontown, the
Tavern was operated by James and Rebecca Sampey and did a lively business from the time it opened. Four conditions were prevalent at this inn: the place was clean, the food was good, the landlady was civil and her husband was sober.
The building was an inn for only a short period of time from its construction in 1827-8 to 1855 when the importance of The National Road declined due to railroad construction. It was then used as a private home until 1932 when the Commonwealth of PA purchased it and eventually the NPS acquired it in 1961 while making the building part of Fort Necessity National Battlefield.
Today the tavern serves as sort of a museum of what life would have been like traveling the National Road in the mid-19th century. The tavern is open seasonally from April-November normally the same hours as the visitors' center but in particular thanks to the lingering effects of the Sequester, that is subject to change and you should call before coming as the hours are not updated on the nps.gov site.