What is a PDT account reset? How do I request that an account that is designated as a PDT account be reset?

We may be able to remove the PDT flag if you certify that (1) you understand the definition of pattern day trading and (2) you will not engage in future pattern day trading. 

To provide the certifications: Application → Click the "Day Trades Left" and jump to the day-trading page → Click the "Marked as a PDT" → Click the "apply for resetting your PDT status".

Once the PDT flag is removed, you will be allowed three day trades every five business days. If your account gets re-flagged as a PDT account we will not be able to remove the PDT flag absent extraordinary circumstances. Alternatively, you have the following two options

A. Deposit funds to bring the account's equity up to the SEC required minimum of $25,000 or

B. Wait the required 90 day period before any new positions can be initiated.