When you use the word you, you are referring to the person you’re talking to or writing about. You is a noun, and nouns can be countable or uncountable. If you’re talking about a specific noun, the article the should be used. The article the means that you and the people you’re writing/talking to will both know which noun you mean.
The definite article the is also used to refer to a group of nouns that are all related to one another. The group can be a group of nouns that start with the same letter, or a group of nouns that are pronounced the same way when spoken. The group must be distinct from other nouns, which are not part of the same category.
In Early Modern English, there were two forms of the second person singular pronoun you: thou and ye. The latter was generally used to indicate a close relationship with someone, or a deferential attitude towards a stranger or social superior. Due to language change, thou was gradually replaced by you in most contexts, and it eventually fell into disuse altogether, though it is still used in some dialects.
Singular you is now generally considered to be a polite form of address, and it is often paired with the formal you’re in written texts. However, informal you is still common in conversation and can be a sign of respect or familiarity.
In American English, there are a number of other ways to make the distinction between singular and plural you clear: you-all, you-uns, yez and youse (from you + ones) are all used in nonstandard speech, as are yours, a variant of your which is also used by some British speakers. Y’all is a popular choice in Southern United States and African-American Vernacular English, where it is sometimes used to emphasize a regional identity.