You might have been given a special discount code or password by the organiser to give you exclusive access to a ticket type or to reduce the price of a ticket.
To make use of this, just go to the event page and click 'Get tickets' as you would if you were buying a ticket normally.
When the window pops up, click either the word 'Discount code' or 'Password', depending on instructions from the organiser. Then, simply enter your discount code or password in the box that pops up.
A message will then pop up saying 'Discount code accepted' or 'Password accepted'. If you're using a discount code, the price of the relevant ticket type will be reduced. If you're using a password, the padlock next to the locked ticket type will be removed and you'll be able to add it to your basket (or, if the organiser is using a hidden ticket type, it will suddenly appear as an available ticket type).
Simply add it to your basket and complete checkout as normal. If you need assistance with this stage, follow the helpful steps listed in this article.