Think about how books are organised in a library. How do you find the book called High-speed Trains?.
Books on "Trains" should be kept together.
All the books on trains might be in with all the other books on "Transport".
Books on transport will be in "Non-fiction" (books of facts).
All the books for children will be in the "Children's" part of the library.
So books in a library are organised by the sort or class of books they are.
We say they are classified. It makes them easy to find.
Linnaeus classified plants and animals in the same sort of way.
Just like organising books, the idea was to think of what was the same about all the plants or animals in a group.