Think of an account reset as a second chance. In general, a reset is used when you have signed up for a test and things did not go as well as you pleased, or you broke a rule and would like to start over.
By clicking the 'Reset Account' button on the dashboard next to your subscription, you will be given a brand new trading test with the same parameters and rules as your original trading test.
Or you can reset from the account information window:
Please note that this will not change the subscription renewal date, it will only give you a fresh account to work with.