It is very natural for cats to scratch things. Cat scratching behaviour can be interperated in a number of ways for a number of reasons.
Cats will scratch things to keep their claws in good condition. All cat owners should have a scratch post or cat tree in the house to allow your cats to wear down their claws. If your cats claws are too long then you should clip them yourself or take them to the vet or groomers to have their claws clipped.
Cats also scratch to mark their territory. A large deep scratch down a tree in the jungle signals a large tiger and deters other predators in its territory. This may be the same with your cat, if you have many cats in the house it might be a signal of dominance.
Where your cat scratches may be an indication of why they are doing it. If they are doing it on a window or door that goes outside it may be because they want to go outside. It may be them signalling that another cat is outside and on what they feel is their territory.
Your cat is clever, if their bowl is empty they may scratch the kitchen door or cupboard where the food is kept in.
A cat scratching under the stairs door could be a sign of mice. If this is continuous and you don’t know why then you should check for mice droppings. This is the same for a garage door or other area where mice or rodents could live.
Cats will often scratch when they are away from something they want. If your reading a book in bed and you’ve left kitty outside then they will most likely let you know about it by scratching the door, this is there way of knocking and saying ‘Let me in!’