Borrowers are a fictional race of very tiny people, invented by Mary Norton. Norton wrote a series of Borrower books about the adventures of Arrietty Clock, a young Borrower girl who lives between the walls in a human house.
They are called Borrowers because they "borrow" or take regular household objects and use them for their own purposes. So, they use thimbles and spools for chairs and tables, needles for swords, and so on.
Here is a list of Borrower books: