Walk is how smoothly the blade travels through its arc. Talk is the audible snap when it seats fully open or fully closed. Together they are the traditional measure of how well a slip-joint knife is made, and Case is the brand most people judge them against.
A crisp snap means the backspring is properly tensioned and meeting a well-finished blade tang with tight tolerances. A smooth, even sweep with no gritty spots means the pivot and the spring are both right.
It is a genuine quality signal on traditional patterns, because it depends on several things being made correctly at once.
Nothing about the steel, the heat treat or the edge. A knife can have magnificent walk and talk and mediocre steel, or the reverse.
It is also not a safety feature. A slip joint held open by spring pressure closes under enough force on the spine, however loud the snap.
A very strong spring gives a loud snap and is hard on fingernails. A lighter one is friendlier to use. Neither is better and makers tune it deliberately by pattern.
Springs also soften slowly over decades, so an old working knife often has a gentler snap than it did new.