It's the idea that every application can near-arbitrarily choose a bespoke-but-exact mix of versions of every underlying package and assume they all work together. This is same attitude that leads to seemingly every application on planet earth needing to individually duplicate the work of reacting to every single dependabot update for their thousands of underlying packages and deal with the fallout of conflicts when they arise.
Packages in nixpkgs follow the "managed distribution" model, where almost all package combinations can be expected to work together, remain reasonably stable (on the stable branch) for 6 months receiving security backports, then you do all your major upgrades when you jump to the next stable branch when it is released.
Packages in nixpkgs follow the "managed distribution" model, where almost all package combinations can be expected to work together, remain reasonably stable (on the stable branch) for 6 months receiving security backports, then you do all your major upgrades when you jump to the next stable branch when it is released.