Thou, thee, ye [,thy, thine] are variants of 2nd person pronouns.

"You" used to be the formal plural form.

All of these have different connotations and/or subtly different meanings, and context will affect if they are to be taken at face value or are e.g. sarcastic (I've not read enough Shakespeare to recall if he made use of that)

[Thy/thine is your/yours and which is right depends on if it's before a vowel or consonant...]