Definiteness is whether a noun is referring to a specific instance of itself.
The word ‘the’ shows something is definite in English, and the word ‘a’ shows that something is indefinite. Definiteness can be used for nouns that have been referenced in the conversation before, or which are already known to the speakers. The sentence “I saw the dog” indicates that the dog is a specific dog, perhaps one that has been mentioned before, as opposed to “I saw a dog,” which is just a remark on seeing a nonspecific dog that is not known to the speaker and/or listener.
