pH is the measure of acidity or alkalinity of water and is measured on a scale of 0-14 degrees and includes tenths i.e. 7.8 degrees.
Every 0.1 degree is a ten fold change in acidity/alkalinity, therefore a change from say 7.3 to 8.3 is drastic, and if sudden it is a serious threat to your pond life.
A pH value of 7 is considered neutral, below this is acidic and above 7 is alkaline. Ideally a ponds pH should be between 7.0 and 8.0. Many different makes of pH test kits are available which perform numerous pH tests for around £5.