Tolerance? What is it really?

When you build a tolerance does getting high feel any different or does it just take more and more weed to get high on the same level? I’m confused because I’ve seen people I know who have a high tolerance have super red eyes after smoking a lot but still act like it’s not affecting them. I’m confused cuz I smoke on and off so I don’t really have a tolerance personally and never have had one in the past. Explain your personal story with weed below.