"You never know" means that there's a small chance that something could happen. Use this to talk about something that probably won't happen, but is still possible.
A common way to use "You never know..." is to say that something might be useful in the future. For example:
A: Do you want to keep this? We don't use it anymore.
B: Yeah, let's keep it. You never know...
You can also use it when you're hopeful that something great will happen:
I like to dress nicely, even if I'm just going to the grocery store. You never know – I might meet the man of my dreams there!