Two climbers climbing simultaneously, one on the top end of the rope, one on the bottom, clipped into some, often few, anchor points between. Often people will use a shorter length rope for less weight on the leader, and less drag.
If the leader falls, the follower effects a basically standard "catch", but the follower must not fall, as this will pull the leader of, and possibly right into the last anchor point,
HARD. This dictate that a stronger climber may climb as the follower. Sometimes, a " one-way" device is put on the rope, keeping the follower from being able to pull off the leader, in the event of a slip.
I've only simul-climbed a little. Parts of Royal Arches in Yosemite are 5.6 (easy) and under. Speeds things up a lot.