When someone talks about a bike’s geometry, they’re talking about how the bike is set up to fit the rider. Frame size is a huge part of it, but other things like seat height and handlebar position make a big difference as well. People who like to go fast tend to set up bikes aggressively so they can hunker down and be really aerodynamic. People who are just riding for fun (and old fat folks like me), usually prefer a more casual, upright style.
My new bike originally belonged to someone much taller than me. Everything was set to “Big… no, BIGGER.” Today’s lesson (yes, it’s my lesson, not yours) was “if your handlebars are too far away from you’re body, you’re not going to be comfortable.” My stem (that’s the piece that holds the handlebars to the top tube of the fork) is way too large. I did most of the ride with the middle of my fingers on the handlebars. Not comfy.
But other than that the bike rides well. I just need to replace that stem (part is already ordered!) and figure out where to mount a lock and a pump. Then she’s ready for a proper long ride!