pH is the measurement of how acidic/alkaline the pool water is. The pH scale goes from 0 to 14, where pH 7 is neutral. If the pH is above 7, the water is basic and the chlorine becomes much less effective for sanitization; if it is below 7 the water is acidic, and can be corrosive to pool equipment. The optimum pH for pool water is 7.4, since this is the same as the pH in human eyes and mucous membranes. A pH of 7.4 also gives good chlorine disinfection.
So to raise your pH by 0.1, you need to add 40 grams of Soda Ash per 10,000 Litres.
And to reduce your pH by 0.1, you need to add 98.5 Millilitres of Hydrochloric Acid per 10,000 Litres.