What does "emulsify" mean?
When you whisk together two liquids—like oil and vinegar—that normally won't bond together, you can cause them to form a single liquid that may stay stable for at least a short period of time. Think about what oil and vinegar look like when well-whisked - the result is thicker and almost creamy looking: that is an emulsification.
To turn oil and vinegar into astable emulsification (one that will not later separate), you must combine them slowly, whisking all the while, and then add mustard, egg or another ingredient that will keep the emulsification stable.
Recipe: White Wine Vinaigrette