What does the column 'Maint req' mean in TD Ameritrade's position table?

That is the maintenance margin required for that position. Whenever you trade using your margin account, you must (by law, and also separately often by stricter policies from the brokerage) have a certain percentage of equity - at least 25%, often higher. That protects the brokerage from significant losses if your position drops in value significantly (hopefully preventing the brokerage from failing outright in the event of a major collapse). If the amount of equity falls below the maintenance requirement, you will have a margin call and be required to put some cash (or equity) into the account to maintain that level.

See Maintenance Margin definition on Investopedia for more information.