It’s a rhetorical construct here: they mean a non-bike thing that also requires scraping your knees to learn.

If the original bike was bike, then the next one is bike' (pronounced bike prime), and the one after that bike'' (bikeprimeprime), and so on.

Compare the orthography to bike_0, bike_1, bike_2, etc.

From the context I would say, the latest fad. The new hot thing, mode, rage, trend?

Probably a reference to “priming” variables in mathematical contexts. You have variable x and then variable x’ comes up. Or you learn to ride a bike and then later on you’re supposed to figure out how to ride a bike’.