When a transmission is slipping it means that the gears are not changing automatically like they should.
If it takes longer for the transmission to change gears when you accelerate then your transmission would be slipping.
A common cause of transmissions slipping is low transmission fluid level. Transmission slipping could also be due to some more problems inside the transmission and if you don't add transmission fluid and continue to drive the car while the transmission is slipping then you risk having your transmission go out completely.