Once you have built up this amount of Renown you will start getting offers from kings interested in having you as a vassal refusing an offer lowers that king's reputation of you, and if it falls below 0 he will no longer offer vassalage. In order to join a faction you will need to build up at least 150 Renown raising your relations with the king will allow you to join with a lower amount of Renown(5 less renown needed for joining per relation point).
