Du, dir, dich all mean “you” in English, they differ in German because they are in different grammatical cases.

Du

• Nominativ case
• Used as the subject of a sentence, the one who is performing the action
• e.g. Du singst. (You sing.)

Dir

• Dativ case
• Used as the indirect object of a sentence, the recipient of the action, or with certain prepositions that require the Dativ case.
• e.g. Ich gebe dir das Buch. (I give you the book.)

Dich

• Akkusativ case
• Used as the direct object of a sentence, the one directly receiving the action, or with certain propositions that require the Akkusativ case.
• e.g. Ich sehe dich. (I see you)