Oil and water naturally repel each other. Emulsion is a term for a combination of oil and liquid that are bound together so they don't separate.
When you make a vinaigrette salad dressing, you can see that the vinegar and oil separate. If you shake it well, they combine for a while, then separate slowly.
Emulsifiers are things that keep them together longer. Emulsifiers include egg and prepared mustard.
Oops, clicked submit before I was done. So then, if you add a little prepared mustard to your vinaigrette and shake, you'll see that it takes much, much longer to separate - i.e., stays in a more stable emulsion thanks to the action of the emulsifier.