A lot of cities and neighborhoods have leash laws that prevent dogs from being off their leashes in a public area. Apparently, this cat has appointed itself as a vigilante enforcer of said laws.
Sammy the dog is just trying to enjoy his day at the park and, like a good dog, is trying to run over to his owner who is calling to him. Unfortunately for him, this cat's got its paws gripped tightly on the other end of his leash and refuses to let go. Poor Sammy tries his best to walk away, but it's all in vain. In the battle between canine and feline, the cat clearly wins this round.