Huh? Whu? You lost me there Keeper! What gives!? [confused]

Don't worry if you're unfamiliar with this term "Opacity Stop", it's not really addressed like that in practice. Just in the manual. ;)

In the real world, we'd simply refer to it as "transparency".
So the above title would then read more familiar to everyone, "Using Transparency in your Gradients".

This is a great function in Photoshop and can be quite useful in a variety of ways. One of which is to create specialty Clipping Masks, for unusual image blending effects. (see sample image below)
[list]For example... how long would it take you to create a bullseye that has alternating black and transparent lines? It would take less than a minute using Opacity Stops, and applying a Radial gradient to a new layer, above your image. This would free you up to focus on your effect, instead of labouring over making the clipping shape.
After which, you could select the shape, convert the selection to a Path, and save this special shape as a new "Custom Shape". (under the EDIT menu)[/list:u] Below is a sample image showing what a gradient using transparency would look like in the gradient editor. Note that the arrows seen above the preview strip are used for the transparency settings, while the arrows below the strip represent the colours being used in the gradient. You can have as many of each type of arrow as you wish. To add new transparency arrows, simply click the mouse above the preview, at any point along the gradient. Then below the preview strip, change the settings to suit your need.

White arrows mean the opacity at that point in the gradient is set to 0% (completely transparent). While a black arrow denotes an opacity of 100% (no transparency). In our example image, this creates a blend of transparency into white; going from left to right.

NOTE: Remember... if you have any questions/comments about this or any other tutorial in this board, please post to a new thread within the "General Photoshop" board.