What Is the Convection Equation Really Defining?

The most common letter used in all of PDEs is u. So it's u sub something and u basically represents your solution. And the problem with that is that u is like very different in everything, right? It represents something completely different in most equations. Sometimes you can tend to again get lost in the math and just think, okay, u is just a solution. But no, what is it really representing? Well, completely different things in completely different scenarios.