When you buy a property there is legal work that needs to be done. You will often hear this called conveyancing.
You will probably use a solicitor to do this work for you although you can use a licenced conveyancer.
Your legal bill will be the fees for the legal work plus other expenses that your solicitor has paid on your behalf, such as searches and Land Registry fees. You may see these additional expenses described as disbursements.
Some remortgage deals may include free conveyancing, otherwise expect to pay around £500 + VAT for the legal work plus the cost of disbursements.