A rondel is a circular track where each space represents a different option. Players choices are restricted by having to move around the rondel, which means players are restricted from making the same choice over and over again. A key element to a rondel, as I understand it, is that it is a cycle. The choices are in a specific order. So a game like Puerto Rico or Alfred's Wyke where you simply can't take the action that the last player did doesn't count.
With some games that are considered rondel games, the rondel seems to be about gaining resources like Finca while others are choosing actions like Santiago de Cuba. One game that I'm not sure counts as a rondel game, Vikings, the circle is just price control for the market.
My problem might come the fact that there seem to be two definitions for rondel. One is for games that use cycles of actions or resources. The other definition is for games that are part of a specific series by the designer Mac Gerdts. (I don't know if he coined the term or if he set out to make a series of games using rondels or if the series was declared after he made a bunch of game using them)
Which might be part of my problem in a couple different ways. I haven't played any of those games. In fact, the only Gerdts game I played was one play of Princes of Macho Picchu. Although Antike Duellum is on Yucatá so this might give me the push to learn it.
So, on the one hand, I haven't played the games that helped define the concept of rondels. And on the other hand, the term have been overused too broadly.
Not grokking rondels hasn't hurt my experiences as a gamer but it does sometimes make me feel like I've missed something crucial. At the same time, Boardgame Geek doesn't list it as an official mechanic so I might not be alone.