Myrtle saw Tom driving Gatsby's car while they were in New York. Myrtle is trying to run away from her husband, who she thinks is going to try to take her away from all of her friends, because he believes she is having an affair, which is true. She runs out and sees Gatsby's car, and thinking that it is Tom driving, she tries to get the car to stop. Sadly, it is Daisy, who is driving the car, and she is so upset, that she strikes and kills Myrtle.
When Gatsby realizes what has happened, he wants to say that he was driving the car, not Daisy. This is a pivotal look into the character of Gatsby. His love for Daisy is so great, that he is willing to take the blame for Myrtle's death so Daisy won't have to face any consequences for her actions. It is ironic that Daisy kills her husband's mistresses, and doesn't even know what she was to Tom, and that it was done with Gatsby's car, whom Daisy has also been involved with.
This scene sets up the actions for many people. In the end, it is because of this accident and Gatsby's car, that will eventually lead to the tragic outcome for Gatsby.