what does it mean when people say the universe is flat?
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You can imagine a 2D space being flat (like a piece of paper, the top of a table, or anything people usually call flat) or curved, like the surface of a curved object (a sphere for example). One way to check if it’s flat, is that the angles of a triangle must add to 180° and the circumference of a circle must be 2πr. If it’s curved, it’s not the case.
In the same way, a 3D space can be flat or curved around a 4th inaccessible dimension. The way to check if it’s curved is to check the angles of a triangle, the ratio between the circumference of a circle and its radius, or some other things.
The 3D space is approximately flat around low gravity objects, and curved around high gravity objects like black holes.
The observable universe (if you check as far as you can from Earth in all directions) is perfectly flat at the precision we can observe it. So if we check a circle or triangle on the CMB (lights coming from the farthest we can see, which is also the earliest we can see) it is flat.
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