The definite article in English is the." There are many situations in which the is used, but generally the is used when both the speaker and listener have a specific thing or person in mind; they are both thinking about the same thing or person.
The is used a second time a speaker mentions a noun (called 'second mention'). Eg. Yesterday I saw a dog. The dog was running after a cat. The cat was running after a mouse. The mouse ran into a hole. The hole was very small. (The dog = the dog that the speaker was talking about in the first sentence, the dog that the speaker saw yesterday. The cat, etc.).
The may be used with any noun as in (1) the student, (2) the students, (3) the water. In (1) we have a singular count noun, in (2) a plural count noun, in (3) a noncount noun.