The option to book in a trial lesson gives you and your students some flexibility to have an initial lesson or meeting before starting regular lessons together. This can give your student a chance to meet you and get to know your teaching styles. At a trial session, you can prepare a full lesson or you can simply use it as a chance to discuss the student's learning goals and create a plan of action for future lessons - make sure to have discussed this with the student before the session to ensure you're both on the same page.
The cost of a trial session is up to you - this varies between tutors. Some will offer a trial session at a fixed rate, with a minimum price of £5, whilst others may offer a 25% discount on their usual hourly rate. There is no requirement for you to offer a discounted first lesson, but the option is there if you would like to do so. Whether you choose to, or choose not to, offer a trial lesson is completely down to you.