Generally the difficult decisions like run outs, stump outs, boundaries are given to Third umpire. The Third umpire will sort out the decision and helps both the umpires who are on the field.
But there is another umpire who helps these three umpires both on the field and off the field. He is Fourth umpire.
If any of the umpires on the field gets ill or needs any other help, then the Fourth umpire will replace him.
When the umpires on the field want to change the ball, then they take the help from Fourth umpire. You can see on Television screens, Fourth umpire carrying a bunch of used balls and helping out the field umpires in selecting them.
The conditions and measurements of stumps, boundary markings, crease markings, sight screens are managed and inspected by Fourth umpire one day before the Match. He also makes ground staff to ensure the rollers on demand.
The behaviour of players will be regularly visualised by the Fourth umpire. It is the Fourth umpire who communicates or informs the information regarding the behaviour of players to Match referee. Then depending on the given information the Match referee takes a disciplinary action on players.
The other very interesting factor is, it is the only Fourth umpire who can move into players dressing room without any permission. Generally players get upset or feel unhappy with some of the decisions given by umpires. They may express their feelings on the field against the umpire which is not good in sport. And it is not correct in spirit of the sport according to ICC rules.
These players may express their frustrations more in their dressing rooms. This drama will be watched by the Fourth umpire who can move freely in their rooms without any permission.
Then simply he informs the matter to the match referee. In one way the Fourth umpire plays a character of detective in the stadium.