Inflation is the general rise in prices over time. When there is inflation, the same things cost more money than before — not because they have become more valuable, but because money is worth less.
This happens for several reasons: governments print more money, production costs rise, or there is simply more demand than supply. Moderate inflation (1–3% per year) is normal and even desirable. When it spikes, it becomes a serious problem.
One of the main reasons people invest is precisely to beat inflation: if your money does not grow at least as fast as prices, each year you can buy fewer things with the same savings.
In 2010, filling a car's tank cost about $55. Today it can cost $88 or more. The car does not need more fuel to run — fuel simply costs more money. That is inflation in action.