In the 9th Century AD, a Greek monk named Cyril travelled to Kievian Rus with the goal of spreading the Christian Bible. As he met with this primitive slavic nation, one thing stood out as a clear obstacle to his goal: the slavics didn’t have any system of writing! So Cyril made up a new alphabet for their language, still called “cyrillic” to this day. This alphabet was based mostly on the Greek alphabet. However, Russian has more sounds (and different sounds) than Greek, and there weren’t enough letter in the Greek alphabet for all the Russian sounds. Therefore Cyril borrow letters from Hebrew.

Why Hebrew of all languages?
Because the only way that Cyril was able to talk to the slavics was through Hebrew, with Jews serving as translators. Cyril knew both Greek and Hebrew (as Hebrew is the original language of the Bible!)and the Jews in Eastern Europe at the time knew both Hebrew and Slavic. So of course, after Greek, Hebrew was the next logical language to pick letters from!

On another note (and to respond to Emily), I know of cases when Spanish learners would say “embarasada” thinking it meant embarassed, when in reality it means pregnant. So they’d wonder why they were being congratulated all of a sudden.