Mask
A tool or layer that defines which areas of a composition are visible and which are hidden.
In practice
A mask controls visibility without deleting anything: some areas of the mask let the layer show, others hide it, and gray areas make it partly visible. Because the original pixels stay intact, the decision can be refined or undone later. A designer might mask a city photo into the shape of large headline letters, or use a soft-edged mask to fade a product photo gently into its background.
Often confused with Clipping Path
A mask can be soft and partial, painted in shades of gray; a clipping path is a hard vector edge where each area is simply in or out.
Try this
Place a photo behind a large bold word and use the word as a mask so the photo shows only through the letters.