pH is a measure of how acidic or alkaline a solution is, measured on a scale from 0 to 14 with low numbers being most acidic and high numbers being most alkaline.
The most common example of an indicator is universal indicator, which gives the colors we've shown on the pH scale ranging from deep red at a very low and acidic pH to yellow green and then bluey purple as we increase the pH and it becomes more alkaline.
The benefit of the probe compared to an indicator is that it can be much more accurate and precise because it doesn't involve humans guessing shades or particular colors.
An acid is any substance that forms aqueous solutions with a pH of less than seven, as acids release hydrogen ions in water which make the solution acidic.
Neutralization reactions can also be represented in terms of hydrogen ions and hydroxide ions, where h+ from the acid and oH- from the base combine to form H2O and because the acid and base have both been neutralized in these sorts of reactions the pH of the products should be seven because they're neutral.