A slider is a rectangular piece of fabric with a grommet at each corner, threaded onto the canopy's lines. It sits at the top of the lines during deployment and slides down as the canopy inflates, controlling how quickly that happens.
The single component that made modern canopies practical
A ram-air canopy without a slider would inflate almost instantly, producing opening forces high enough to injure. The slider is held up by air pressure as the canopy starts to spread, restricting how fast the lines can separate, and descends progressively as pressure equalises.
The result is an opening spread over a few seconds. Slider size and configuration are among the factors that determine whether a canopy opens softly or briskly.
Related terms
Slider up and slider down describe configurations used in different disciplines — BASE jumping sometimes uses slider-down for very low deployments. A slider that fails to come down after opening is a situation covered in training.
These pages explain what things are so the language makes sense. They are not instruction and cannot substitute for it — skydiving is taught in person by rated instructors at a drop zone, and anything involving equipment handling or emergency procedures belongs there rather than on a web page.