Traffic limit is the gigabytes of transferred data you pre-pay for at the beginning of the billing period. Every month the system checks if you stay within the limit, and if you don't, it charges you for the excess. Then your traffic is reset. Control Panel navigation is not included into the total traffic.
Total traffic limit is the amount of traffic you pay for in advance. It is charged at the recurrent fee. However:
You can always buy additional amount of the resource in order not to be charged the extra payment. The system e-mails you a notification once you have used 80% of the traffic limit.
Example: If your billing period is 2 months long and started on the 5th of October, the first billing month will end on the 5th of November, and your total traffic will be reset. However, if you change your total traffic limit on the 15th of November, your traffic will be reset and a new billing month will start.
If you expect to run up more traffic than comes with your plan, it's a good idea to raise your traffic limit.
To change the traffic limit:
Read more about traffic.