Rocket League is a game where you hit a ball with a car (Basically soccer with cars)
There are other things you can do in this game such as Opening Crates or playing the other "Sports" or gamemodes. They also have something called a "Rocket Pass" which when you tier up gives you customizable items that make your car look better! .
The other Sports/Gamemodes consist of "Hoops" which is basically a basketball version of rocket league where you hit the ball into the hoop. Another is Snow Day which is just a hockey puck and you hit it into the goal.
Then there is Rumble, Rumble gives you a random powerful every 10 seconds and you can use them to Hurt the other team or help yourself
The last one is Dropshot where you have to hit the ball into the ground of your opponents side and hit it at a spot twice.