It’s the pH of the liquid that comes out of your plant’s pot after you water them. This liquid has a pH that correlates, but does not quite exactly measure the average pH of the grow media (Soil, coco coir, peat, etc) in the pot.
Most of the time, we test for runoff pH to gauge fertilizer building up in the soil, which causes it to get very acidic as well as exert osmotic pressure on the plant. Measuring the input and runoff EC is a much better way to tell. If the runoff has a higher EC than the input, you have fertilizer buildup.
Check the runoff pH to see how bad it is and if you need to do anything about it. You have a grow media that you want at a specific pH for optimal growth, and you should be feeding the plants with a nutrient solution that is near that value.
In an ideal world, your first few feedings will be within 1 pH measure of that target, and after a few weeks will converge on that number. If this happens, good.