Shutter speed
How long the camera keeps the light in for one frame: fast freezes the action, slow lets moving things blur into streaks.
See it
What it is
Shutter speed is how long the sensor stays open for one frame, written as a fraction of a second: 1/1000, 1/60, 2 seconds. Short means motion is frozen mid-air, every splash droplet a hard sphere. Long means anything moving smears into a streak, which is how you get silky waterfalls, light trails on a highway, and star trails.
It is one leg of the exposure triangle, so it never moves alone: halve the time and you halve the light, which you pay back with a wider aperture (less depth of field) or higher ISO (more noise). Rough anchors: 1/500 and up freezes sport, 1/250 handles a walking person, 1/60 is about the floor for handheld with a normal lens, anything slower wants a tripod. Video has its own rule, the 180 degree shutter, where you set the speed to double your frame rate (1/48 at 24fps) so motion blur looks the way audiences expect.
Gotcha: two different blurs get confused. Subject blur (the subject moved) and camera shake (you moved) both come from a slow shutter, but only shake is fixed by stabilization. Also in bright daylight you cannot always get slow: the fix is an ND filter, which is sunglasses for the lens, not a lower ISO you have already bottomed out.
Ask AI for it
Render this scene as a long-exposure photograph at roughly 1/4 second on a tripod: static elements razor sharp, while everything in motion (passing people, traffic, flowing water) dissolves into smooth directional streaks. Keep the streaks continuous and soft-edged, not ghosted duplicates. (For the opposite, ask for a 1/2000 second freeze: every water droplet and hair suspended mid-air with zero motion blur.)