Obviously English is not your first language :). Hammock is not the name of the person, but it's this (so called) bed that you see on the beach stretching between two trees.
Now to answer your question: a hammock activity is a task or a group of tasks that stretches between two other activities. Hammock activities are flexible, the beginning of the hammock activity may change as well as its end (so, unlike other activities, a hammock activity is not time estimated). Let's assume that Task A is supposed to end on Monday, April 25th, and Task E is supposed to begin on Friday, May 6th. A hammock activity, consisting of 3 tasks (B, C, and D) begins when Task A ends, and end when Task E begins. So task B begins on Tuesday, April 26th, and task D ends on Thursday, May 5th. Now if Task A ends on Wednesday, April 27th, then task B begins on Thursday, April 28th. This is where the similarity between a hammock activity and a real hammock is prominent (it's the flexibility!), a real hammock, even if the trees are (reasonably) far from each other or very close, can still be installed.
Hammock activities are common in construction projects, where an activity has to happen between two tasks. An example of such activity is drainage.