Interpolation is the process of filling in the unknown data between two known values. In digital video and film, this usually means generating new values between two keyframes. For example, if you want a graphic element to move fifty pixels across the screen to the left in 15 frames, you’d set the position of the graphic in the first and 15th frames, and mark them both as keyframes. Photoshop interpolates the frames between the two keyframes. (Interpolation is sometimes called tweening.) Interpolation between keyframes can be used to animate movement, opacity, styles, and global lighting.

In Photoshop, the two types of interpolation are linear interpolation and hold interpolation.

Linear Interpolation

Creates an evenly-paced change from one keyframe to another. Changes created with linear interpolation start and stop abruptly at each keyframe.

Hold Interpolation

Changes the value of a layer property over time without a gradual transition. This interpolation method is useful for strobe effects, or when you want layers to appear or disappear suddenly.

If you apply Hold Interpolation to all keyframes of a layer property, the value of the first keyframe holds steady until the next keyframe, when the values change immediately.