Your credit score is like a grade the bank gives you. Bad grade = no loans. Good grade = easy loans.
What they look at to grade you
Whether you pay your debts on time. If you have many debts open. If you've ever missed a payment. It's like a history of whether you're trustworthy with money.
How it affects you
Good score = get credit easy and with low interest (3%).
Bad score = can't get credit or it's expensive (8-10%).
The difference on a mortgage is brutal. It could be 100,000 euros more expensive over 30 years.
How to fix it if it's bad
Pay everything on time. Don't ask for lots of new credit. Reduce what you owe. Wait. Over time, your score improves.
The biggest mistakes
Not paying bills. Applying for 10 cards in a month. Having unpaid debts. Letting something go to a collection agency. Those destroy your score.
It's slow to fix
You can ruin it in a year, but it takes 5-10 years to recover. That's why don't ruin it from the start.
You don't need to be rich
To have a good credit score, you just need to pay your bills on time and not borrow too much money.
Practical advice
When you apply for credit, read it. Understand what you're signing. If you can't pay back that money in the agreed time, don't borrow it.