To "bail" means to leave. To "bail on" means to back out of something previously planned or to let someone down.
"Are you leaving so soon? We're just starting to have fun."
"Yeah, I know it's early, but I've got to bail. I promised my wife I'd be home for dinner."
"I know I promised to pick your in-laws up at the airport tomorrow, but my boss just told me I have to work all day tomorrow."
"Oh, don't bail on me now, I can't leave them stranded at the airport!"
"I'm sorry, but there's nothing I can do about it."