To transliterate is to rewrite something in a different alphabet. When you transliterate the name Пётр from Russian into English, it's generally spelled Peter.
Transliterate comes from two Latin roots, trans, or "across," and littera, "letter or character." It's related to translate, with an important distinction: when you translate something, you interpret its meaning and put that in a different language. When you transliterate, you're simply changing the alphabet in which a word is written, so that it can be read or pronounced in a different language.