If you buy a candybar from someone, you give them money and they give you a candybar.

If you buy a house from someone, you give them money and they give you a house.

• If they still owe money on their mortgage, then they use your money to pay it off. If they have money left over then they made a profit. Hooray for them!
• If you took out a mortgage to afford the house, then you now owe the bank a whole lot of money. The house itself is the guarantee that you will pay them back: if you don't, they take the house.
• When you sell the house, the buyer will give you money. If you still owe money on your mortgage, then you will pay the bank. If you have money left, you have made a profit. Hooray for you!