What is the meaning of はあ?

mhm; yes (used to express polite agreement)

um; uh (used to express thought, confusion, or uncertainty)

huh? hmm? what?! (used to forcefully call attention to something)

oops (used in response to a failure or mistake)

ha! (expression of loud laughter)

A sigh.

The sound of heavy breathing when sexually aroused.

Source: wiktionary.org