If you get an offer to buy your home and you're happy with it, your solicitor will send a letter called a 'qualified acceptance'.
This letter tells the buyer you accept their offer, and lists any conditions you want the buyer to agree to before you'll go through with the sale.
Your solicitor and their solicitor will then send each other a series of letters called 'missives'.
These letters are used to negotiate the conditions you and the buyer have, so you can both come to an agreement.
Once you're both happy with the conditions, a 'concluding missive' will be written. This is a binding contract between you and the buyer.