A pH calibration buffer solution has a known and fixed pH-value and is used for calibrating pH electrodes this should be done reguarly to ensure the meter keeps its accuracy.
Just fill some of the fluid in a clean measuring cup and use it to calibrate your pH meter.
The specific instruction on how to calibrate is to find in the manual of your pH tester.
After a calibration please discard the used solution. Do NOT pour it back into the bottle as this may contaminate the whole bottle and make the solution inaccurate.
Bottle of 300ml