When is the best time for my toddler to start schooling?

I've seen schools that offer classes for toddlers as young as 2 years old. During my time, nursery starts at 4 years old, and I think that would be a good time for a child to start schooling as they can communicate well already, and understand instructions. The schools that offer classes for 2 year old children, what do they teach them? I'm not talking about playgroups or anything like that. They call the classes pre-nursery.. Like a preparation for nursery. But what exactly do they teach to prepare them for nursery? Aren't they a bit too young to understand and remember?