If you don't know much about Montessori, here is my "in a nutshell" version: It's a method of schooling usually associated with preschool and elementary education. There are mixed age groups (e.g. 3 to 6) in classrooms. There is at least one adult who gives one-on-one lessons on how to work with "materials." The materials do the teaching, not the adult.
There are hundreds of different types of materials. They're designed to teach or exercise a particular skill, but they look like games and are all designed to be "beautiful" as to entice children to them (Montessori is really serious about this; materials are not allowed to be broken or chipped or worn). They follow a particular progression. Children may work with any material for which they've had a lesson. They can work with it as many times as they like.
My favorite example of how Montessori works is how it develops reading skills. First a child is introduced to materials that involve very short crayons - when they use them they strengthen the muscles in their hands. Then they're introduced to letter-tracing materials. Then they're introduced to letter/sound matching materials. Children who follow these lessons wind up writing first, and then reading follows very naturally. The emphasis on developing physical capabilities first really demonstrates the attention to detail that's typical of Montessori.
The math education progression is impressive also. There are materials that have children doing proto-multiplication, exponentiation, algebraic manipulation, and more. My partner developed a material that teaches counting, addition, and subtraction in base 8. (Her 4 and 5-year-olds understood it much more quickly than her colleagues!)
There are parts of the method I don't like (e.g. dogmatic resistance to rewarding performance, for example), but overall it feels like a huge improvement over typical early childhood education.