Soil pH is the measure of the acidity or alkalinity in the ground, numerically equal to 7 for dirt with a neutral pH, increasing with rising alkalinity and falling with increasing acidity. The pH scale commonly in use ranges from 0 to 14.
Soil pH is not fixed; you can take measures to alter it. If soil pH needs to be lowered (i.e., the earth isn't acidic enough), apply commercial fertilizers containing sulfur / ammonium-N. Ammonium sulfate is such a fertilizer. If soil pH needs to be raised (i.e., the earth isn't alkaline enough), apply lime.
Fortunately, the Web has online calculators to help you determine how much lime or sulfur you would need to apply, given the size of the area where you are attempting to raise or lower soil pH. You can find such online calculators here:
Why Is Soil pH Important?
Now that we've gotten the basic information out of the way, let's dig deeper into what "soil pH" means -- i.e., what it's true significance is to those who garden and landscape.
Soil pH is not, itself a nutrient, but it relates to plant nutrition. That's because it governs the availability of nutrients to plants. Particular nutrients that a plant needs can exist in the ground in abundance, but if they are not made available -- due to conditions that are too acidic -- they will do the plant no good.