Kaizen actually generally means "improvement." Toyota and others (DevOps) used the term in a different context, eg, kaizen and the improvement kata. I'd suggest a couple books that have really shaped the DevOps movement (and that talk about this concept in detail):
Phoenix Project (one of my favorites) and the DevOps Handbook (a fabulous integration of concepts and real life examples).
I also recommend Jennifer Davis and Katherine Daniels' book, Effective DevOps:
And don't forget The Goal by Eliyahu Goldratt:
The idea of improvement in DevOps is that it is equally, if not more, important to spend a portion of your time improving your systems than just doing work. We have all been in the situation where a process isn't automated, but yet we still push forward doing it the hard way, because "we don't have time" to do it right. Well, we should spend (maybe 25%) of our time actually improving our workflow so these bad habits go away.