Creating your own ticket type allows you to manage the different kinds of tickets your helpdesk is used for. You may create a ticket type for support requests, one for sales enquiries and another for internal staff HR requests or any other type of incoming issue that your business needs to track and manage.
For each Ticket Type you can create a different form depending on what information needs to be gathered in order to support the request. Support Requests may need to ask the user for the Urgency or Impact, but a Sales Requests might not. An HR request might need a dropdown to select a date range or a free text field to collect a body of information.
On the main Ticket Types listing page you'll see a list of all available Ticket Types for your system. Everyone starts with a single default Ticket Type, and you can add more as required. See Managing Ticket Types for more detail.