Definition
A clipping mask is a graphic editing technique that uses the outline of a paired shape or piece of text as a frame, so that the image or color grouped with it shows only inside that frame. You don't have to cut the original image with a knife. It simply looks as if the image has been slipped into a window of whatever shape you want.
How a photo slips into a round frame
Say you want a nice round profile photo. You don't need to snip off the corners of a square photo one by one. Just draw a circle underneath, place the photo right on top of it, and apply a clipping mask.
The computer then treats only the area of the circle below as a clear glass window. The photo is still its original square shape, but only the part that overlaps the circle is let through to your eyes.
The biggest advantage is that the original stays untouched. Nudge the photo sideways or shrink it, and the round window stays just as it was, so you can adjust the composition freely any time.
One catch: the stacking order depends on the program. In Photoshop and Figma, the shape goes on the bottom and the photo on top. In Illustrator it works the other way around, and the shape that acts as the frame has to sit on top. InDesign skips layer order altogether and has you paste the photo inside a shape's frame.
To be more precise: how it differs from a layer mask
When you work in design programs, you'll run into a similar feature called a "layer mask." Both hide parts of an image, but they differ clearly in how the frame is made.
A layer mask works by painting in black and white on top of a single photo. You decide with a brush "which parts become how transparent." Areas painted black are hidden, as if erased, and only the white areas show clearly.
A clipping mask, on the other hand, borrows the form of a separate shape layer or text layer. With no brushwork at all, the photo is instantly locked into the frame of a star you've already made, or the shape of bold lettering.
The trick to filling letters with a photo of space
On movie posters and magazine covers, you often see bold lettering packed full of a sparkling night sky or a city skyline at night. That's a classic use of a clipping mask.
Type the word you want in big letters on a text layer, put a photo of the Milky Way on top, and apply the clipping mask. That's it. The space photo fills every stroke of the letters, and stylish typography is done in seconds.
You can also stack several photos, shadow effects, and gradients on top of a single frame. Change just the one base shape at the bottom, and every design element above it rearranges itself to fit the new frame at once, which saves a huge amount of work.
Common misconceptions
When you apply a clipping mask, the parts of the image that stick out past the frame are permanently cut off and deleted.
The original image data is still there. If you release the mask or move the image around inside the frame, the hidden parts can come back into view at any time.
Where you meet it
In Photoshop or Figma, you use it to fit a portrait neatly inside a circle and make a profile icon.
You use it to fill the thick strokes of a poster's title letters with a texture image, such as the sea or flames.
In one sentence
A graphic technique that uses the outline of a paired layer like a window, so an image fits neatly inside the frame without being damaged.