Engrams are gem-like, polyhedral things that are used to deliver rewards in Destiny 2. They're basically mini-loot boxes: Do a job, get some engrams, go to The Tower to decrypt them, and surprise! It's your 27th Khepri's Horn. You'll pick up lots of them as you play, and most of the stuff you get from them won't be terribly valuable.
But some engrams are more powerful than others, and will drop items that are actually useful for increasing your Guardian's Power level—a central part of the game. It's all a bit complicated, but the quick-and-dirty of it is that bigger numbers are better, and powerful engrams are good—and you can get a free one from the Bungie Rewards program.
One note for Destiny 2 newcomers: Because of the way the game handles the Power grind, you should probably hold off on claiming the engram until you hit the "soft cap" of 900. That's as high as your Power Level can get through regular gameplay; after that, the only way to increase your it is through powerful reward drops, like this one, which can take you to the hard cap of 950. The engram's power level is set based on your Power when you claim it, so if you grab it before you hit 900, it won't actually help you on the climb to the hard cap.
If you're just getting started in Destiny 2 and this all seems new and weird, we can help. Our Destiny 2 Beginner's Guide will give you the lowdown on the different classes, gear types, activities, and how to best power your way through the Power Level grind.