In a nutshell
Inflation is a rise in prices (and therefore a decline in purchasing power) over time. In other words, if the average price of a basket of selected goods and services increases over time, then a unit of currency (e.g., 1 British Pound) gets you less than it did before.
What causes inflation?
When demand exceeds supply, prices tend to increase as consumers compete for limited resources.
Production costs for business increasees leading businesses to push some of the burden to consumers by increasing prices.
More money is in circulation (maybe the government prints more money or lower interest rates lead to more borrowing) which leads to increased spending which leads to businesses raising prices to cash in.