It's just a way of describing the way a knife feels and sounds as it is being opened and closed. I would think it is most applicable to slipjoints, since you are opening and closing against the pressure of a backspring. A good "walk" is a smooth, consistent feel of the pressure of the backspring against the tang as you are moving it to the open or closed position. You tend to feel this in your fingers as you hold the blade while opening it, and in the palm of your hand against the spine of the blade as you are closing it, or against whatever surface you use to close yours.
The talk is the click of the tang run-up against the end of the backspring when it fully opens, but moreso that little "clack-ping" sound as the kick strikes the backspring when closing. Unless you ease your blade closed all the way, it's from when you get to that point where the friction against the tang is overcome by the backspring to snap it to that final closed position.
Best way to understand it is to open and close some different knives, and pay attention to how it feels and sounds. You can tell the difference between different knives, designs, and makers. And at some point you discover that there is a certain combo of feel and sound that is more pleasing to you than others. That's when you discover what "good walk and talk" means.
Square-tang knives (half-stops) have a significantly different walk and talk than cammed tang knives, so almost need to be compared to each other as separate categories. And you may find that you prefer the walk-and-talk of one category over the other.
It's hard to describe to someone without sitting down with a handful of knives and showing someone what you mean, knife by knife.