When we say FORK, that means a new coin is being produced ( which is basically derived from a mother-coin that existed already ) in order to improve some parameters (that seem to be lacking in the previous coin ) such as : reducing fees & accelerating the confirmation of transactions process, coming up with new features...etc
BTW, if you want to know more about Eth forks that were planned to be achieved in 2019 , you find here a sufficient description of the different forks ( Eth classic vision, Eth nowa, and Constantinople)