She yells through the door at him that she’s making the video and just wants Leonard to know that she left him because she liked him so much that she knew she’d end up marrying him, but she didn’t feel ready for that step, so letting him go was the better option.
Zack boots up the computer and shows the video to Penny and Leonard.
And then, because Penny was drunk, she passed out on the keyboard and, assumedly, woke up without remembering the video, which is why she never sent it to Leonard.
In it, Penny is clearly drunk, explaining why she doesn’t remember making the video, and as she begins to tell Leonard in the video that she wants him to understand why she broke up with him, Sheldon knocks on her door.