Shoshin means "beginner's mind". It's about approaching life with an attitude of openness. Looking at the world with fresh eyes. Putting aside the believes you have, freeing yourself from limiting preconceptions.
There are many times where we are biased before experiencing something new. We have read or heard about it and so we already created an opinion that might influence how we experience this new thing. Now imagine experiencing the same thing through the eyes of someone who had no idea what to expect, that's beginner's mind.
Imagine driving to work. You've done this so many times, it's almost on autopilot. You aren't noticing all the small things you were looking at when you first made this drive. If you take the time to look at it with fresh eyes, you'll probably find out you missed some things that changed since that first drive.
There's a great Zen story about this:
A student went to see a Zen Master to gain knowledge and deepen his learning. He asked a lot of questions and then went into a detailed explanation of everything he understood. The student spoke so much, the master wasn't able to offer his wisdom.
While the student was speaking, the Zen Master started pouring cups of tea. One for himself and one for the student. The student's cup filled to the brim and the Zen Master kept pouring tea while spilling all over the table.
The student yelled: "Stop! The cup is full, there's no room for more!"
The Zen Master replied: "Like this cup, you are full of your own opinions and speculations. How can I teach you Zen, before you first empty your cup?"