The answer is that anyone who doesn't have the full value of the property they are buying available to them (which is of course virtually every first-time buyer) needs a mortgage in order to be able to buy the house.
A mortgage is simply a type of loan, and a loan that is repaid over a very long period of time: indeed the typical lifetime of a mortgage loan is 25 years.
Because they are lending you so much money, the banks don't do this without wanting some security in return - and the security is simply the home: if you fail to keep up the repayments on your mortgage then the bank has the right to repossess the property, which is why it is so important to ensure that you only take out a mortgage that you can afford to cover.
So... how much can you borrow? Well to find that out you need to meet with a mortgage advisor at a bank, and they will come up with something called an agreement in principle, and this will tell you how much (in principle) they would lend you... ti is based on information that you give them: such as your income, your expenditure per month (to give an idea of how much spare money you have per month) as well as looking at your credit history and also any savings that you have.
The agreement in principle gives you a good idea of what sort of mortgage you can get, and therefore help you work out what price range of property purchase is realistic for you. Remember of course you'll also need the deposit of typically 10% the value of the property too... the more you pay off in terms of the deposit, the better the mortgage deal you can get.
Whilst it can seem quite daunting that you will have to pay back so much money, and of course with interest much more than the amount of the mortgage you take out (perhaps double or so) remember that the repayments are spread out over 25 years, and that the majority of people have a mortgage: so don't panic too much, that is perhaps one reason why it is called a mortgage not simply 'massive loan' or similar!
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