Some water is slightly acidic, even fancy bottled water can be acidic (the marketing is not always true). You can get a testing kit from some drug stores to test the acidity levels of liquids and other substances. Good quality water can offer alkalinity however, it can be easier and more potent to get a dose of alkalinity going with strongly alkeline liquid like lemon waer, green tea or coconut water.
The general principle here is to make your body more alkeline because most peoples bodies are waaay to acidic and if he Ph levels were to be corrected, many diseases like inflammatory related conditions, would dissolve.