Gō-no-sen 後の先 : To respond to an attack after it is initiated.
Sen-no-sen 先の先 : To anticipate an attack and to take the initiative.
Sen-sen-no-sen 先先の先: To anticipate and attack before the attack.
In a martial sense, it takes years of training to be able to understand the fine line between Sen-no-sen and meaning of Sen-sen-no-sen. We learn the basics of technique applying the Gō-no-sen concept t beginner levels (low to mid level kyu belt), and if we are good we might advance to being capable of responding sen-no-sen at 1st kyu & shodan levels. Sen sen no sen, requires high proficiency of zanshin. If zanshin is not involved, then you either risk the possibility of not seeing the intent of attack and being hit, or you're going around punching people in the face for no reason.
Sen means "before". It is also a shortened form of sen-te (literally "before hand") and typically refers to "an attack" in budo terms. Go means "after".