What does the phrase "hide the implementation from the user" mean when doing composition in Java when the user won't see your code anyway? [closed]

I am currently learning Java, and have recently started composition, yet every now and then I hear or read the phrase "hide the implementation from the user" when using different design patterns, what does this mean? Isn't the developer the only individual able to see the code? What does it mean to hide the implementation from the user? I've recently learnt interfaces so maybe interfaces could be referenced for any answers to help me understand what this means, thanks.