pH probes require periodic calibration. Calibration is done by placing the probe tip in a small amount of buffer solution of known pH and then adjusting the pH meter until the pH value displayed matches that of the pH of buffer.
The photo shows two different types of pH buffer: a single-use packet and bottles, from which small quantities (about 5 ml) are withdrawn for calibration. The single-use packets are almost certainly more accurate, but they're also more expensive: The bottled buffers were $5.00 each at a local hydroponics shop. The packets sell for about one dollar each. In my experience, the bottled buffers work fine.