Locking transparency simple prevents pixels that are 'clear' from being changed.
Say you create a new layer. It's empty... or 'clear' as though it were a transparent plastic sheet laying over the background.
Now take a brush and dab paint in a small spot in the middle of that layer. All the areas outside the paint are still 'clear', and the background will show through.
Suppose you want to spray paint over that paint dab now. What will happen? The spray will go on the paint dab, and all over the clear plastic too. Then the background is obscured by all that over spray.
This is where the 'transparency lock' comes in. With the lock enabled, the spray paint ONLY is allowed to hit the paint dab... and NONE goes into any 'clear' pixels.
This works for all the brushes and tools, and effects too. You can edit away on any painted pixels on that layer, while never worrying about over shooting onto the clear pixels.
If you want to make it even more interesting... the 'lock' also will protect pixels that are 'partially' clear! So if you paint a dab of transparent paint, so you see your color, but the background still shows through it, that level of transparency will be protected while you work away with other tools and effects.
Hope that helps.
EDIT:
Why did the noise filter not change the 'clear' parts of the layer? Because some filters work by adding lightness and darkness to existing colors on the layer. In the clear areas, there is no color to change, so there is no result from the filter.