From my experience, I’ve only done one where I had to explain my code, they just picked apart my code and asked me what that certain section was supposed to do and why it was necessary or if it could’ve been done in a better way. It’s sort of a way for them to make sure you know what you are doing and didn’t just copy paste from GitHub or Stack Overflow to make something work. But as long as you can explain it you should be good to go. Being that it is a Junior position I don’t think they are going to be to hard on your code, depending on how long you have been programming that is.
In terms of questions, it’s going to vary between companies. Mine didn’t ask extremely difficult questions, mainly they asked simple questions to make sure I knew the basics and, again, being that it is a Junior position I can’t imagine they are going to throw a curve ball at you.