A safe word, when used properly, creates a situation where you are free from worry. The bottom can truly let go, scream, cry, whimper, moan, fight, even say "no no no no no!" and none of these things will stop the action, when they truly wants it to continue. The Top can say and do whatever they like, teasing and tormenting with promise of release or even harsher punishment, knowing that the bottom knows they are not going to stop unless either party safe-words.
When safe words are used effectively, an environment of incredible trust can be created. Boundaries and limits can be pushed and explored knowing that in extreme situations, and extreme situations only, the action will be stopped with one word.
The "Universal Safe Word" system invovles using a trafic light analogy.
Most BDSM practitioners recognise this system, so you can normally rest assured that if either partner in a scene says RED, the action will stop immediately.
Sometimes, a safe sound or action may be needed for instance when someone is gagged. Some people will use a series of grunting sounds. Others will hold an object in their hand, and dropping it is their safe signal. I personally like using bells, as an aural signal is very hard to miss.
Be aware that there is never shame in safe-wording, a scene is not a trial of endurance (unless this is previously agreed as the scene's purpose!)
Also, a Top can just as easily safe-word as a bottom. Safe-words are all about staying safe, so if the Top feels that the scene is heading in the wrong direction, or it is making them feel unhappy, it is only responsible of them to safe-word out of the situation.
Safe-words go hand in hand with good communication; Before, during and after your scene.