The Elaysians are a humanoid species. They are accustomed to low-gravity.
When exposed to high gravity, they need physical support to function. Long-term support for the higher gravity is provided by a wheelchair or an anti-grav unit.
Dr. Julian Bashir devised a medical treatment based on neuromuscular adaptation. The Elaysian would be able to travel in Earth-stle gravity, however, the process couldn't be reversed.
Only a handful of Elaysians have left their planet. Melora Pazlar became the first Elaysian to enter Starfleet Academy.