Walk and talk describes how a traditional slip-joint knife opens and closes: the walk is how smoothly the blade travels, the talk is the audible snap when it seats fully open or fully closed. It is the classic measure of how well a traditional knife is made.
Good talk is a clean, crisp snap, not a mushy click. It comes from a properly tensioned backspring meeting a well-finished blade tang with tight tolerances and no slop.
Good walk is a smooth, even sweep with consistent resistance and no gritty spots. Together they tell you the spring, the tang and the pivot were all made and fitted properly.
It is not a safety feature and it is not a lock. A slip joint held open by spring pressure can be closed by force on the spine at any time, however crisp the snap.
It is also partly a matter of taste. A very strong pull gives a loud snap and is hard on fingernails; a lighter pull is friendlier to use and quieter. Case knives are the reference point most people judge walk and talk against.