The game is pretty ridiculous. You have a team of up to four players who don't always know each other, and friendly fire is always on, so half the time you're getting accidentally blown up or shot by one of your own team mates as opposed to by the enemies, which can create some funny situations. But the theme of the game is that these soldiers are just cannon fodder in humanity's war against the "enemies of democracy" (bugs and robots) which need to be "liberated." So every time you die, you just immediately drop back into the fight as another nameless Helldiver, only to be squashed or shot or blown up again thirty seconds later (if you're lucky enough to last that long).
The harder the difficulty level you select, the more enemies the game throws at you, so you have to work as a team if you want to complete the mission and make it to the extraction point at the end. But working as a team is quite difficult due to the chaos going on. Plus the Helldivers themselves are almost comically clunky to move around -- they're slow and heavy, they can barely jump, and they can dive to the ground, but if you do that you only dive a couple of feet and it takes a few seconds to get up again.
It creates a lot of what they call "emergent gameplay" moments where the sandboxy-nature of the game mechanics creates unique situations. Here's a gameplay video of a few newbies getting into the game:
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