ph basically means the ammount, measurment or levels of acidic compounds in your soil (ph is also a factor in many different things, such as pools and what-not but I figured I answer the question as most questions here go). ph affects the health of your crop, by allowing your plants to intake much needed nutrients. if it measures too high or too low it not only is basically an indication that your plants enviroment isn't safe for them but also increases insects, etc.
Ph is a scale from 1-14 that measures balance between alkaline and acidity concerning organic compounds,or liquids,,,cannabis grows well between the 6.5 - 8 range,,,,,7 bieng neutral..
Here's a basic guide for pH levels in soil and soiless. Keep in mind that a pH too high or low will lock out essential nutrients.