system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What does it mean when physicists say the Universe is flat?

Currently in an astronomy class i took for fun (not nearly as fun as i thought), but many interesting concepts come up such as this one. How is the universe flat?
gpt
Essentially, it means that the universe mostly follows the principles of Euclidean geometry except in localized areas.

That is to say--a triangle light-years across will have internal angles that add up to 180 degrees. If the universe was not flat, this would not be true and what appears to be a straight line would actually be a curved line. 

More practically--this means that if you look and see a star off in the distance, you can be reasonably sure that the star is actually in that direction and the light isn't following a curved path to get to us.

Gravity, particularly extreme sources of gravity, can warp space-time into non-flat sections; that's where you get stuff like gravitational lensing.