Resolution indicates the maximum amount of pixels that can be shown on a digital device's screen. The resolution is typically given as two units, with horizontal pixels followed by vertical ones: for instance, 1,920 x 1,080px.
In general, the display resolution doesn't need to match the resolution of an image: you may still be able to watch a video shot on 4K UHD resolution (3,840 × 2,160px) on your lower-resolution smartphone, TV, tablet or monitor, just as you could still watch a video with a resolution of 480 x 360px. Yet, both circumstances require scaling and optimization processes that an older device may not be able to perform. Whichever the case, bear in mind that the image you'll see on the screen won't exceed your screen's maximum resolution and that bigger screen size doesn't necessarily bring about more resolution.
You should also be aware that, although resolution, display resolution, and screen resolution are the most common terms to specify an electronic device's pixel dimensions, some would say that calling it "resolution" is a misnomer. The reason is that pixel density is a more accurate indicator of a screen's sharpness, as it tells us how many pixels exist within a given area (usually an inch). Puzzled? Don't worry: we've also written about pixel density. All things considered, both resolution and pixel density are critical factors when picking a display device.
To say we've come a long way from the first phones to the smartphones available today seems like an understatement. Less than 20 years ago, we were playing the Snake game on phones with 48 × 84-pixel displays (like the legendary Nokia 3310), and we can now watch movies on gadgets with resolutions of up to 2,160 x 3,840px – such as the current resolution frontrunner, Sony Xperia Z5 Premium. It feels impossible to guess the numbers we'll be looking at in another two decades!