It's essentially a teaching style that's very popular with rich people. The grounds of it are that they let kids explore creatively and learn themselves without much direction. They're expected to do things that interest them and learn the skills they want to learn rather than being sat down and taught things, IIRC. It aims to make kids independent at a young age - for instance, in the morning, you would let your child choose what clothes to wear without any direction or changes (so they could choose a thick wool jumper on a very hot day for arguments sake). One of the key things is that everything should be accessible to the child from an early age. They also advocate putting mirrors right next to mattresses from a very young age so that baby can see themselves and use it to self soothe, I think (they don't tend to use cots).
It's where all the bland wooden toys with no paint on come from, because bright colours are too stimulating or some bullshit. II don't think they do discipline either, there's no 'No', but redirection or 'positive' disciplining.
Loads of videos on youtube about it. On the surface and in some circumstances it can seem really good for a bit, but my opinion is that it's only good up to a point and can be used alongside other styles of parenting - there are reports of kids who were raised in Montessori schools can't read or write because they were never pushed to do that kind of work.