"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!