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: there is more money in circulation than the economy is growing (banknotes are issued by each country's central bank, and most money is created by banks when they grant loans), production costs such as energy rise, or there is more demand than supply. Low, stable inflation is normal: most central banks aim for prices to rise by around 2% or 3% a year. 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.
Fifteen years ago, filling up a car's tank could cost about $66. Today, in many countries, it costs $99 or more. The car does not need more fuel to run — fuel simply costs more money. That is inflation in action.