Command & Conquer Rivals is a new C&C for the more modern era, and by that I mean it’s for mobile. Being shown at E3 actually upset a few people since cellphone games usually aren’t shown at the event. For a more in-depth explanation for why you will see more of it tho, look here. Are Mobile Game Players Gamers?
As for what the game is, well its C&C. You gather resources and fight over a Nuclear missile in the center of the map. Whoever controls 2 of the 3 spots when the meter fills up shoots the enemy base, whoever hits the enemy base twice wins. Pretty simple concept really. The game will be available on IOS and Android but to my knowledge is currently only available in pre-alpha on Android. The game is fun but does have its issues. While feeling decently like C&C they make it a point to tell you that for now there are no in-game purchases, meaning the full game will have some.
You do occasionally win boxes with money and troops, and it is a save assumption these are what you can buy. If that is the case, this is clearly going to be a pay to win mobile game from EA. To make matters worse, at least for now the game seems to be nothing but PVP with no way to practice or even grind for stuff. The game however is fun and easy to get into. Best wishes and may the gaming gods bring you glory.