Ready? Here we go!
At its most basic, an emulsion is a suspension two liquids within each other that would not naturally mix.
Think of a liquid--a cup of vinegar, for instance--as made up of millions of tiny droplets. If you pour oil into the vinegar, at first the oil will float on the top of the vinegar because it's less dense. However, if you whisk them together, the tiny droplets forming each liquid start to mix together and become suspended within each other. This is an emulsion.
However, this simple vinaigrette will eventually separate back into vinegar and oil because, at a chemical level, there is nothing holding the drops of each liquid together except for the temporary confusion of having been whisked together.
To get a stable, permanent emulsion, you need to use something to hold the drops of opposing liquid together and prevent them from separating. This "something" is called an emulsifying agent. And this agent is like a mutual friend who holds the oil-based liquid in one hand and the water-based liquid in the other. It creates a chemical bond with each liquid and becomes a bridge between them.
Up next week: why sauces break and how to fix it!