Loan to value is the ratio between the size of the loan you are seeking and the mortgage lender's valuation of the property.
So, if you're borrowing £55,000 on a property valued at £100,000, the LTV is 55% and the lender will feel comfortable you have enough equity in the property for it to be reassured that if you stopped making your interest/capital repayments, it could sell your property and recoup the money you owe.