Test performance indicator

A metric, in general high level, indicating to what extent a certain target value or criterion is met. Often related to test process improvement objectives, e.g. Defect Detection Percentage (DDP).

In simple English:

Any measure to reflect the quantity or quality of the testing done measured over a period of time. 

Day for example you are trying a new diet to reduce weight. You would weigh yourself before, during and after the diet to understand if the diet is working. 

Similarly when you start a testing project you decide on certain parameters to measure over a period of time. You would expect these parameters to help you judge if your testing process is working towards the goals you want it to achieve.