To place him in perspective for millennials. When John Glenn went into space we were still somewhat unsure if a person could survive in zero gravity at all. When he went into space it was on top of a converted ICBM, built under the direction of Werner Von Braun a former Nazi and member of the SS. Both the US and USSR "Russia" our cold war rival picked the brains of German rocket engineers.
While he was not the first man in space, nor the first American in space, he in Friendship seven, the particular Mercury capsule he was in was the first American to Orbit the Earth. This was a crucial first step to our eventual moon landing, and all manned space exploration since then.
Being under 40 it can seem like space travel was a made up thing. I once had a student tell me they thought the "moon man" statue of the MTV video music awards was "conceptual artwork". They were 100% serious... and had no idea the moon landing was a real actual thing (or even a conspiracy). That is not their fault but a failure of the educational system and of our country to let manned space exploration of other celestial bodies slip once more into the realm of sci fi fantasy.