Right Concentration is the eighth branch of the Eightfold Noble Path and involves the practice of Jhana meditation.
I understand the concept of a concentration practice, but dang official Jhana meditation is complicated. The basic practice itself seems to start fairly simply, with concentration on the breath (as a side note, this is essentially zazen meditation), but as it progresses you are moving through a lot of different objects of concentration and there is a high degree of systematizing of various mindstates that arise (i.e. am I in First Jhana? Second Jhana? etc.).
From a non-Buddhist perspective this is extremely esoteric. I can see why mindfulness practice is extremely widespread while Jhana meditation is not. It's just flat out complicated, and far more embedded within the Buddhist structure itself.
This is about as accessible a conversation I have heard about Jhana meditation and the book is good as well.