It's a single-player survival, bullet-hell roguelite. Visually, it reminds of those Flash games back in the days.
Gameplay loop is quite simple: move around the map killing swarms of enemies, collect XP, level up, choose a skill or upgrade, rinse and repeat. Try to survive as long as you can. Eventually, more enemies spawn but you also become more powerful.
It's very addictive, though. One of those "one more quick run before bed" and you realize you're still playing two hours later... I sometimes start a run while my code compiles or during those endless online meetings where I'm not concerned but still get invited to for some reason.