What are the fundamental sauces that every cook should know how to make?
The next answer is a simple pan sauce.  

After sauteing a protein in a pan, there are caramelized bits of fat, spice, and flesh that make for a great sauce base.  

Getting those bits into a sauce takes a little work, but it's easily accomplished by adding an acidic liquid to the pan and allowing it to deglaze, or breakdown the fatty bits into the sauce.

You can deglaze with red or white wine, stock and lemon juice, or alcohol (be careful if it's high proof, since it will flame).  After adding the acid, stir to break up the bits and let simmer for long enough to reduce the liquid about 30 to 50 %, longer for thicker sauces.

Then add fat, either butter, oil, or cream, and whatever seasonings (tomato paste, mustard, Worcestershire sauce, etc. you want in the sauce). Simmer for just long enough to incorporate the ingredient (1-2 minutes max).

This is a good technique because it can work with any pan-cooked dish and whatever ingredients you have around.

The basic steps are the same, but the results are varied and quite tasty.