"To be set in stone"

Here is an expression that I heard the other day, and I thought it was a common one that probably doesn't make it into the textbooks.  

Any idea what it means?  

Here's an example of it used in a sentence: ""I'm thinking about going out with some friends tonight, but nothing's set in stone yet."

Here is a video that explains the expression and provides a few examples.  

https://youtu.be/gXENIW99ZeU

<a href="https://youtu.be/gXENIW99ZeU" style="color: rgb(17, 85, 204); font-family: arial, sans-serif; font-size: small; line-height: normal;">TO BE SET IN STONE</a>

I'm curious, do you (non-native English speakers) think that you would be able to get the meaning of this expression just from the context of a conversation if you heard a native speaker use it?  

Do you have a similar expression in your language?

I look forward to your responses.  :-)