Usually refers to Conservative backbench peers in the House of Lords who rarely make an appearance, except when forced to do so by the party whip.
Examples of debates that achieved high turnouts of backwoodsmen include the introduction of the Poll Tax, 1988, when Margaret Thatcher used a three-line whip to force backwoodsmen to turn up en-mass. The uniquely unpopular legislation (by 20th Century standards) was rammed through the Chamber with little debate. Other events that occasion high turnouts include anything about fox hunting, and of course motions to get rid of Hereditary Peers.
If any of this terminology sounds dodgy, Tory peers do have that image.