Remember when a burger cost 2€?
Now it costs 5€. The burger didn't improve. Everything just got more expensive.
That's inflation.
Simple definition
Inflation is when prices go up every year. Everything costs more.
Why does it happen?
Many reasons. Basic concept: if banks print money without creating things, money is worth less.
Real example
Your grandmother bought a car for 10,000€ in 1990. Today it costs 30,000€.
Not better. Just 30 years of inflation made everything expensive.