If one looks at the microscopic structure of cartilage one will see that it is composed primarily of a slippery white 'matrix' (imagine a contact lens - flexible and slippy but strong) which is strengthened by the incorporation of randomly arranged collagen fibrils (tiny fibres).
Within the matrix are a relatively few cartilage cells ('chondrocytes') and they are suspended, apart from one another, within small 'bubbles' called lacunae within the matrix.
If hyaline cartilage is damaged, it will not be easy for the cells to multiply and cover over the defect because they are trapped in their lacunae. So if a chunk of cartilage is knocked off in an injury, it really does not heal properly. And likewise if the cartilage is worn down with arthritis. The best the body is able to do is to send in cells from the bony layers under the cartilage and try to replace the damaged tissue - but it can only do this slowly and only with a second grade type of cartilage called 'fibrocartilage'. Fibrocartilage has many more fibres in it, hence its name.
The cells that are pulled in to do this job are 'stem cells', primitive cells that can transform themselves under different circumstances into different types of specialist cells, and some of the techniques of cartilage repair involve drilling or poking down into the bone marrow within the defect to allow these cells to migrate and transform.