The word you is a second-person pronoun that can refer to a single person (singular) or more than one person (plural). As the only second-person pronoun that can be both singular and plural, you is an essential part of our language. It is also unique in that it can be used to make an object of a verb or to make a predicative complement.
The first use of you as a singular pronoun dates from Old English, when it was pronounced yeow and yowd. It was replaced by the more formal you around the mid-16th century. Thee and thou continued in polite if informal use as personal pronouns, though they were sometimes used to show deference or superiority to strangers or social inferiors. They began to lose respectability as singular you became more common, leading to the rise of special plural forms like youse, you-uns, yousies, and yez, which never fully replaced the regular singular form.
In modern English, you can function as an object of a verb in the present indicative, the perfect indicative, and the past indicative: You are going to school today. You washed the car. You went to school. You washed the car.
It is also possible to use you as a subject of a sentence in the subjunctive, the conditional, and the imperative: You might go, you might not go, you might not go. You can also use you as a conjunction: You can, you could, and you might, which are all examples of auxiliary verbs.
Aside from the auxiliary verbs, you can also be used to make adjectives: You are red, and you might be blue. You can also combine it with another noun or noun phrase to create a compound noun: You are so busy, and you might be tired.
You can also use it to make a noun phrase in the accusative: You are annoying, and you might be rude. You can even use it as an adjective: You are obnoxious, and you might be loud.
The most common way to use you is in the plural form you guys, which is often shortened to just youse. It is a common informal expression that can be heard in many regions of the United States. Younger speakers and some politically progressive speakers are more likely to perceive it as gendered and problematically noninclusive of trans and nonbinary people, so they might avoid its use. Plural yous is a more acceptable option, but it can be perceived as unnecessarily formal and stuffy in some contexts. The letter u can also be used to replace you in certain informal contexts, as in I miss u!