Something where you achieve something and it is robust, such that, you could rebuild if necessary. People often think of it as increasing the current state, which is incomplete. Also incomplete is increasing the potential top state. Part that is left out is increasing the bottom state. I think that you introduce a new (higher) floor that is fairly robust.
Once you learn to walk, for example, now you can open up to do a lot of other things, but you also are unlikely to go back to not knowing how to walk. There is incremental improving, standing, scooting, crawling, but walking is a substantial level up from those.
For life, there are moments of leveling up, which are related to realizations put into action. Actionable wisdom. Robust systems. Think of a person who has built up his or her own business or way to earn money in a good way. You could probably drop him or her into an english speaking country, wipe the bank account, or have just enough to live a year, and in 5-10 years, he or she could likely be rich again.
They have leveled-up. For me, being a wage earner is great, but I am seeing that the next level up is having equity in business. Income producing assets. Then that produces wealth.
Then, not after, but while that, the realization that happiness is not material things, but something else. You figure and learn the skills for the something else. Being loved, being in the sun, smiling, laughing, those can be parts of those things.