There are generally three price indexes to measure inflation rate: consumer price index, producer price index and GDP price conversion index. Simply put, when the government issues too much money, prices will rise.

Data inflation in a broad sense is generally defined as: under the credit currency system, the amount of money in circulation exceeds the actual needs of the economy, which leads to the devaluation of the currency and the overall and sustained rise of the price level.

In Keynesian economics, the reason is that the change of total supply and total demand in the economy has led to the movement of price level. In monetarist economics, the reason is that when the amount of money in the market exceeds the amount of metal money needed in circulation.

References:

Baidu encyclopedia-inflation