Filter media is the material that traps the muck. Sand is the classic. Glass media is a step up in fineness and it tends to last well. Cartridge elements are a paper or polyester concertina you rinse. DE powder coats a grid and filters very fine particles.
None of them is magic. A dirty filter, a pump that is too small, or a pool full of leaves will beat any media. On a fibreglass pool the water is already easier to keep clear, so we usually spec a media that is simple to service on a residential pad.
If you are replacing an old sand filter, it is a good moment to talk about glass media or a cartridge unit that fits the same space. We can look at your equipment when we quote a new pool, or when we are on site for another job. The goal is clear water with a routine you can keep.