One of the ways in which inflation is measured is called the Consumer Price Index (CPI). The changes in prices of a selected basket of consumption goods are monitored over time to determine how the purchasing power of the national currency changes.
Purchasing power is a financial ability of consumers to buy goods and services with a unit of their home currency. For example, if in 2018 for $1 you could buy 1 kg of apples and the year later for the same dollar you could only afford 0.8 kg of the same type of apples, your purchasing power has decreased. For the same amount of money, you can buy fewer goods. Purchasing power has declined due to inflation or change in the price of goods and services over time. Apples now cost more, their price has increased.