Beginner's mind is when someone pretends that they don't know anything about something they are already familiar with in order to learn new things about it. It's like when you start playing a new game and you don't know the rules yet, but you're really excited to learn. Even if you already know some things about the game, you try to forget them for a moment and think like you're still learning.
When we use a beginner's mind, it helps us to be more open to new ideas and to see things in a new way. It helps us to not make assumptions or jump to conclusions. It's like when you first start learning how to read and you sound out every letter because you don't know which words are familiar yet. You don't assume you know what word a group of letters make just because it looks similar to something you've seen before.
So, in short, beginner's mind means pretending you don't know anything about something so you can learn more and be more open to different ways of thinking.