"Hump day" is an informal way to refer to Wednesday. It is often referred this way because in a typical Monday-through-Friday work or school week, Wednesday is the middle day. Once Wednesday's work is over, one has gotten over "the hump" and is closer to the next weekend than the previous weekend.
"Hump day" has been used to refer to Wednesday for years, but it has experienced a surge of popularity, as proven by Google Trends; "hump day" peaked as a search term during September 2013. A series of humorous GEICO ads in which a camel walks around an office on Wednesday, asking all of his co-workers "Guess what day it is?" aired during May of that year. This likely accounts for the surge in popularity.