A load is a single flight of jumpers to altitude. Drop zones run loads through the day, and getting on one means manifesting — registering with the desk that assigns people to flights.
The language around it
Manifest is both the desk and the act: you manifest for a load. A load call is the announcement that a load is boarding, usually with a time — 'twenty minutes' meaning twenty minutes to boarding.
Load organisers are experienced jumpers who plan and brief group jumps, particularly at larger drop zones and at boogies, so visitors can jump with people they have never met.
How aircraft shape the day
A turbine aircraft carrying twenty jumpers to 14,000 feet turns around quickly. A Cessna carrying four to 10,000 feet means fewer people per flight and longer between jumps. Both work; they produce quite different days.
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.