Many cats, especially longhairs, can get matts. Matts are when chunks of hair on the coat clump together such that the hair almost feels ‘glued’ together (similar to a dreadlock in human hair). These clumps of hair usually occur when a cat isn’t properly groomed and are unfortunately very difficult to undo.
The best way to get rid of a matt is to slowly try to separate the hairs in the clump. You can do this using a comb and your fingers. Depending on how bad the matt is you may or may not be able to do this. If you can’t comb through the matt you will need to cut that part of the coat out.
The best way to deal with matts is to prevent them in the first place. You can do this by regular grooming of your cat.