Standing room- people pay to stand in the back, so they don't technically have seats. Over 100% percent means every seat was filled and there were people standing.
^^^ Fun Home is currently the highest percentage on the most recent grosses @ 103.5% which makes sense considering it won Best Musical and is in a non-traditional house with well under 1000 seats.
It's a bit more complicated than just adding standing room. Gross potential does not include premium pricing, for example. So if a show is selling a lot of premium seats at high prices, this increases the gross compared to the potential.
CZJ at opening night party for A Little Night Music, Dec 13, 2009.