I seriously doubt anyone actually has to "jump" onto or off of anything. He's probably talking about a large plywood box on the trailer, meant to simulate high loads. You get up there and practice your lumber-tarping skills.
As far as lumber tarps go, the ones I used were somewhere between 100 and 120lbs. Occasionally, when they get wet, that'll add another 20lbs or so of water. I'd say about 99% of the time, the forklift operator will lift the tarps on the load for you.