Your preschooler is learning really fast now. You would sometimes get really surprised by his abilities and level of understanding. He is now able to look at 2 different objects and find similarities and differences in them. This level of understanding of properties of different things helps him in categorizing. Give him a set of toy cars and ask him to separate them into big cars and small cars and he is able to do so easily.
What you need to know
The ability to categorize is important at this age as your child will be encountering many new objects, events, people, etc. on a daily basis. If they are not able to categorize these then each one will look novel and they will have to respond to it anew. The ability to categorize is an important cognitive development that is fundamental to further understanding of mathematical concepts.