macros?

"macros" is short for "macronutrients", which means the major nutrients: protein, fat, carbohydrates. When people speak of their "macros", I find they are actually referring to the target levels of macronutrients in their diet.

It's short for macronutrients: fats, protein, and carbohydrates. From the standpoint purely of reaching or maintaining a particular goal weight, they're not important. It's your balance of calories consumed and used that affects whether you're gaining, losing, or maintaining a particular weight (barring unusual medical issues). However, there are lots of reasons (from maxing protein for building/maintaining muscle to counting carbs for diabetes to counting fats for high cholesterol, to a whole range of life-style and popular diets based on the latest best-selling books) to try to hit certain goals for macros. MFP has default percentages for macros, and most users seem to agree that the protein default is too low, but you might find disagreement over whether fat or carbs should be reduced to make room for more protein.

Carbs, fat, protein, and alcohol. All food is made up of them and they are all slightly different. Enough fat is essential to make your body work properly, particularly hormones etc. Enough protein is essential for general bodily functions and repair, as well as if you want to build or retain the muscle you have. Carbs aren't "essential", but they are used for energy, so the more you have the better you will feel (unless you have too many in one go and feel sluggish).

Ooops, forgot alcohol! (Which MFP doesn't track, but you still want to log what you drink, because it will track the calories from alcohol.)

I had figured out what macros are, but can someone explain the easiest way to calculate what to put in if you don't want to use the MFP default? I tried the other day and it made my head spin...

?? Do you mean the process for changing the defaults, or what your particular percentage goals should be?
The process is to go to "my home," then "goals," then "edit goals," and I think you want to start with whichever macro is listed first (I think it's protein) and work your way down, because MFP automatically changes the macros listed below the one you just edited so that they all add up up to 100.

If you mean what your particular percentages should be, that's going to depend on your goals and any medical issues you have, and I wouldn't advise just adopting the goals recommended by some random stranger on the Internet.

Try going on IIFYM.com