pH stands for "Potential Hydrogen" and is a measurement of how acidic or basic the water is in your pool. pH should be tested daily during startup or season opening. Once you gain experience with your pool, less frequent monitoring may be appropriate depending on your pool's typical rate of pH change. pH levels between 7.2 and 7.8 are suitable for swimming, while levels between 7.4 and 7.6 are ideal. pH below 7.2 tends to make eyes sting or burn. pH below 7.0 can cause damage to metal parts, particularly pool heaters with copper heat exchange coils. High pH can lead to calcium scaling. pH contributes to the CSI, which indicates the tendency for plaster damage or calcium scaling. Aeration will tend to cause the pH to rise. You can help the rise of pH by lowering Total Alkalinity (TA).
Most pools will drift to a higher pH. Especially with pools using salt chlorine generators, new construction of a pool or a fresh coat of plaster.
You can raise pH with borax or soda ash/washing soda. Soda ash/washing soda will increase TA more than borax will. You can lower pH with muriatic acid or dry acid. How much you will need for a given pH change depends on several other numbers, most importantly TA and borate. Higher TA and borate levels require more significant chemical addition to change the pH.