pH is a measure of hydrogen ion concentration in a solution and is mathematically given as:
`pH = -log_10 [H^+]`
where, [H+] is the concentration of hydrogen ions. The pH scale is a scale with numbers from 0 to 14 and is a logarithmic scale. Based on the pH scale, we classify substances as acidic, basic (or alkaline) and neutral. A pH of less than 7 indicates an acidic solution, while a basic solution contain pH of more than 7. Battery acid, vinegar, etc. are acidic; in comparison, sodium hydroxide, ammonia, etc. are basic in nature. As the pH increases, concentration of hydrogen ions decrease and those of hydroxide ions increase. On the other hand, a decrease in pH value, indicates increasing hydrogen ion concentration.
Acidity and basicity are measures of ability of a substance to donate and accept protons (hydrogen ions). Many people also relate the pH scale to acidity and basicity to quantify proton loss or gain capability. Higher acidity means more capacity to donate protons, the more protons that one can donate, more acidic it will be. Similar is true for basicity.
Among drinks, lemon juice, orange juice, tomato juice have high acidity (denoted by low pH); water is at a neutral pH, while black coffee has a pH of around 5.
Hope this helps.