A mortgage is a loan the bank gives you to buy a house. You pay back that money over 20, 30, or 40 years. That's it.
How much money the bank lends
Usually 80% of the house's value. You put down 20% (your down payment). If the house costs 200,000, the bank lends 160,000 and you put down 40,000.
How much you pay each month
You divide what you owe by the number of months. But you also pay interest because the bank needs to earn money. So your monthly payment is money you're returning + interest.
The interest rate is your enemy if it's variable
If your mortgage is variable, the interest goes up and down with the market rate. When rates go up, your payment goes up. When they go down, it goes down. It's unpredictable.
Fixed-rate mortgage: predictable but expensive
You pay the same amount every month. It's nice knowing exactly what you'll pay. But it's usually more expensive than variable.
What they don't tell you
For the first years, most of what you pay is interest, not principal. You're basically paying the bank, not buying the house. Then it reverses.
It's not a bad idea
Yes, you pay tons of interest. But after 30 years, the house is yours. If you'd rented those 30 years, you'd have nothing.
Be careful not to borrow too much
Don't borrow more than you can afford. The rule is your mortgage shouldn't be more than 30-40% of your salary.