Archaic personal pronoun
Personal pronouns have changed over time, like other word classes - though less than most, because the personal pronouns form a very closed set. One particular example still has influences on Modern English, and surprises speakers of other European languages. This is the use of the second person.
From the beginnings of English up until after the time of Shakespeare (1564-1616), the language distinguished between the singular and plural of the second person - like most other European languages, and many outside Europe. In modern English 'you' remains the only 2nd person pronoun in use, serving for both singular and plural.
In the time of Shakespeare, and other important influences on our language like the Authorised Version of the Bible, 'you' was the objective case of the plural form; the subjective case was ye, and the possessive your. The singular was thou in the subjective form, thee in the objective and thy or thine in the possessive (see the table of personal pronouns).
The use of thou continued after Shakespeare's time in some forms of literature. You are likely to find it in poetry (particularly love poetry) and religious writing (particularly prayers) in later times. It is still common among Christians to address God as Thou in formal prayers, as in church. See also word-endings in older English.