CV stands for Curriculum Vitae. It is a summary of your achievements so far, including any jobs you have had, your qualifications and your skills. It is a really good idea to write a CV if you are thinking about applying to medical school as you can use it to give you ideas when you write your personal statement.
It is also a really good idea to send a CV when applying for voluntary work or shadowing placements, as it can tell the person a lot about you.
Writing a CV is not easy and can take quite a bit of time and effort. There is lots of advice on the internet about how to write a CV. Try the BBC online website as a starting point.