As the name suggests, you take out another, second loan on the same property. There are many reasons to take a second, such as funding other expenses, paying off debts, using it as a down payment, and so forth. Be prepared to go through the same steps as if you were applying for a first mortgage.
You can also take advantage of the extra tax relief: your interest on the second mortgage may be tax-deductible just like other mortgage interest. See our section on Individual Income Tax for more details.