Free Riding in a Cash account means an investor did not pay for the purchase of a security prior to selling that same security. If an investor buys a security and then sells that same security without paying for the security in full by the settlement date, the investor is considered to be in violation of the U.S. Federal Reserve rules regarding trading in a Cash account. Accounts that commit a Free Riding violation will be frozen/restricted for 90 days from re-balancing the portfolio, during which time the account can only make new purchases using settled funds. After the 90 day Free Riding freeze passes, the Cash account can once again re-balance a portfolio with funds from a fully paid position's closing trade as long as that new re-balancing trade is not closed prior to settlement.
The Account Window will display settled funds for new orders in the "Available Funds" section. The "Total Cash" line in the Account Window will include both unsettled and settled cash. The final real-time check for settled funds is done at the Credit Check step when entering a new order.
Find more information on the Free Riding Rule in the IBKR Knowledge Base.