A Free Ride Violation occurs when you buy and sell a security before paying for the position. To better explain this, please see an example below.
On Monday morning, you initiate a cash deposit and purchase ABC stock. Then, on Tuesday morning, you sell the ABC position you purchased the prior trading session. On Wednesday, the deposit you initiated Monday morning is returned/rejected and does not post to your account.
This would trigger a Free Ride Violation since you bought and sold the stock without fully paying for the position.