Montessori is a way of life and raising children, that goes beyond just toys, however certain play materials and play environments from a young age will foster Montessori ideals. There is technically no such thing as a "Montessori baby toy", as it's about the materials/objects used, not specific toys, however, there are Montessori inspired toys, in that they help achieve the outcomes of Montessori learning.
Montessori inspired baby toys are 'self correcting', which means they give immediate feedback whether the outcome of the toy is being achieved or not - in other words, they have a built in error control. The child learns to correct themselves independently and gains problem solving skills. Adult interaction is not needed for a child to work out whether they have completed the task correctly.
For example, while knobbed cylinders are considered a Montessori material, other similar toys are Montessori inspired. Their overall goal is to train the brain to create more organised thoughts and ways of retrieving information.