In any job it is important to have a way of showing people what you can do. It may be that you have physical things you have built, designed or made or that you can show them results you have got for previous customers.
As a graphic designer you need to be able to show your potential customers what types of designs you can do. It may be that you have no qualifications relating to design work and therefore the only thing that potential customers have to go off is your previous works.
The best way to do this is to create a portfolio. If you are a web designer only then you may decide that the best way to do this is through a website. If you also create logos etc. then you may want examples of printed stationery.
When putting your portfolio together do a brief few paragraphs talking about the brief for the project. This will give your potential customer a good overview of the issue the client had and how you overcame it. Try to remember to update your portfolio as often as possible so your latest work is always available to view.