Density altitude is pressure altitude corrected for temperature — in effect, the altitude the air is behaving like. Hot, high and humid conditions all reduce air density, so on a hot day at elevation the air behaves as though it were considerably higher.
Why skydivers care
Thinner air means less lift for the same canopy at the same speed, so canopies fly faster over the ground, land faster and are less forgiving of a mistimed flare. Terminal velocity in freefall also rises.
Aircraft are affected too, needing more runway and climbing more slowly, which is why jump operations at high-elevation drop zones on hot afternoons may run differently.
Where it shows up
A drop zone at 5,000 feet field elevation on a hot day can produce density altitudes well above that. It is one of the reasons visiting jumpers are briefed on the local landing area rather than assuming their home DZ's habits transfer.
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.