Many languages have ways of referring to another person in a respectful way. The English way of using ‘Sir’ and ‘Madam’ is a good example.
One thing that modern English lacks is the twotiered system of the word ‘you’, as used in countless other languages, for example tu/vous in French, ty/vy in Russian, du/Sie in German, tu/Lei (or tu/voi) in Italian, du/ni in Swedish, tú/usted in Spanish, ni/nin in Chinese, and so on. (Linguists use the French as their model and call the two the ‘T-form’ and the ‘V-form’, regardless of language.)
But until a few hundred years ago, English too made this distinction, in separating ‘thou’ (the casual and familiar form) from ‘you’ (the polite and respectful form). The use could not be described as obsolete from Standard English until about 1800, and can still be heard today in certain dialects, such as in Newfoundland, Canada. The familiar/polite distinction can be found in Shakespeare's plays, for instance, where friends call each other ‘thou’, while reserving ‘you’ for superiors:
Thou poisonous slave … come forth!
(The Tempest, Act 1, Scene 2)
I told you, sir…
(The Tempest, Act 4, Scene 1)
It wasn't always so, and usages have glided in and out of fashion over the centuries. To begin with, ‘thou/thee’ was simply the singular form, while ‘you’ was the plural. By the 1500s, ‘you’ had taken over as the pronoun used for both singular and plural (as it is today), and ‘thou/thee’ was only used in certain situations.