Available To Day Trade is the amount of money available to buy securities.
Per Reg T, a pattern day trader is allowed to trade four times the excess of the maintenance margin as of the closing of business of the previous day. This is called Day Trading Buying Power. Overnight margin is 50% (two times). Brokers have the right to impose lower margin limits.
FINRA offers this explanation:
The rules permit a pattern day trader to trade up to four times the maintenance margin excess in the account as of the close of business of the previous day. If a pattern day trader exceeds the day-trading buying power limitation, the firm will issue a day-trading margin call to the pattern day trader.