Your weapon has a superior reach. Against anyone who misses you in melee combat you get a free counterattack. If he makes several simultaneous attacks (c.f. ButtStrike or Lock), all must miss if you are to do this.
As soon as you are hit, you loose this advantage, against any and all opponents. To regain it, you must spend a 3-shot action readying your weapon while unengaged.
Reach does not work against another weapon with Reach, except that the Pike still has reach even against other weapons with reach. Nor does it work in any kind of confined space.
You cannot counterattack against a counterattack.