"Two, six, heave" is a phrase used to coordinate seamen's pulling.
As used by sailors, the person at the front of the team will typically call out the "two, six" part of the chant. During this phase all members move their hands up the line ready to pull. This is followed, in its natural rhythm, by the "heave", called out by the whole team together. At this moment, the team simultaneously lean back on the line, and use their leg muscles to exert a powerful pull upon it. This coordination takes some practice to achieve, but the difference in applied power between a raw group pulling as individuals and a practiced team hauling together is very significant.
There is not a single tempo or cadence for the chant, since this will depend on the task in hand. For example, hauling in the upper topsail halyard will require a long, heavy pull; if the team is not to be exhausted halfway through then the leader must ensure that the pace is slow enough to be maintained throughout the job. Hauling in a clewline, by contrast, is relatively quick and easy, so the chant can be quite rapid. It is also not always necessary to use this method of hauling for the whole of a task; often, the first part of the job can be achieved with simple hand-over-hand pulling, switching over to a coordinated heave for the final tensioning.
After a line on a ship has been hauled taut, it will usually be secured to a belaying pin.
In Britain it has a broader meaning and is often used in any situation where a coordinated pulling effort is required, often where maritime people are involved, but almost as frequently where 'civilians' are working together.