A mockup is the frame of a device photographed from the front, screen left empty, into which you slide a capture. The same screen presented raw looks like just another image; placed in iMac 24", it immediately reads as a computer interface.
This file is a PNG with a transparent background: the device outline and screen area are empty, saving you from any cutting out. You place your capture on the lower layer in Figma, Sketch, Photoshop, Canva, Keynote, or PowerPoint, and the frame overlays neatly, even on a colored background, gradient, or photo.
For a sharp rendering, prepare your capture at the screen ratio, ideally at its native resolution of 4096 x 2304 pixels. The free version is exported in 800 × 721 pixels, more than enough for a screen; the high-definition version goes up to 5989 × 5394 pixels and covers printing as well as tight cropping. Other models are available in the same format, all downloadable for free: iPhone, Samsung Galaxy, Google Pixel, iPad, and laptop mockups.