LTV stands for: Loan-to-Value
Banks and other lenders will extend credit based upon a percentage of the estimated market value of your home.
That percentage of market value minus the amount you owe on your first mortgage (plus any 2nd or 3rd mortgages that you may have) becomes the maximum amount of credit that lenders will give you.
Let's say that your home has an estimated market value of $300,000. The amount you still own on your first (and second) mortgage is $100,000. At 80% LTV, your borrowing amout will be calculated as follows: