To take a simple example:

Let's say a country has only a single 20% tax band that kicks in at 20k.

If you earn 40k, your marginal tax is 20%, but only 20k of your earning is above 20k, so you pay 20% * 30k = 4k in tax for an effective rate of 10% (4k of 40k).

Most countries will multiple tax bands plus deductions which complicates this, and most places very few people have enough of their income taxed at their marginal rate for their effective rate to approach their marginal rate.

Marginal rate is a rate for a band of income. Your effective rate is what you actually pay as a percentage of your total income.

So if you have a marginal rate of 50% but it only kicks in when you earn over a certain amount, you will be paying less than 50% overall (your effective rate).

Your effective rate is the overall average, while the marginal rate is the amount you paid on the last (or next) dollar.

If you made $100k, paid $25K in income taxes, and were in a 33% bracket, your effective rate was 25% and your marginal rate was 33%.