It essentially means something is completely untrue or misleading.
Here are some examples:
* "That politician's speech was full of malarkey. He promised everything but delivered nothing."
* "Don't listen to her, she's full of malarkey. She's just trying to get you to do her work."
* "I'm not buying that story, it's full of malarkey."
Essentially, it's a way of saying something is not to be trusted and likely fabricated.