Game rendering in a strange, low-detail mode

I’m new to UE4, and Unreal Engine in general. When I got UE4 about 3 days ago, rendering my game worked fine. All of the sudden, the game renders in this weird, low-detail mode.
I’m fairly sure that this is due to my low framerate when I test my game, so UE4 puts me in low detail mode automatically.

Is there any way to improve the quality of my scene when I test it? How do I change detail settings?
Any answer is appreciated! Thanks!

I was wondering this exact same thing

You can start by looking at these: