Definition
For a long time, the USGA (governing body of golf rules) allowed players to "anchor" putters. This meant that the golfer used a longer-than-average putter and would "anchor" the butt end of it against their body for additional stability. This is no longer allowed in USGA-sanctioned golf competitions.
Usage
As of January, 2016, anchoring a putter is no longer permitted by the rules of golf, so I wouldn't recommend purchasing a long putter now!