A leader is the term for the setup at the terminus end of your fishing line. Typically a swivel attached to your main line followed by 1 to ten feet of appropriate monofilament, then your hook, lure bait..etc. that would be your "leader". Not sure of your context but for most gamefish to be eligible for record book status, the angler must bring the fish in himself with no assistance from crew. They cannot even touch the rod, reel, anything. So I'm thinking your quote implies the angler had his fish to the boat, was reeled in as far as the leader would permit, and was ready for crew to subdue fish.