To be honest, I've never used this phrase, but I believe it means:
"Be completely honest with me" You use it when you want to change the tone of the conversation suddenly, and become very candid or open with the person.
If you are talking with your friend about something lighthearted, and you really want to tell them something that's been on your mind, you might say "Alright, level with me for a second"
My explanation might be hard to understand. I'm sorry I can't speak Ukrainian or Russian at all. Hopefully, someone who speaks Ukrainian or Russian can explain it to you.