ㅎㅎ

Onomatopoeic. Compare e.g. English haha.

ㅎㅎ • (hh)

1. (Internet slang, text messaging) haha, lol; used to represent laughter, or simply added to soften the tone of the message

   Synonym: ㅋㅋㅋ (kkk)

• ㅎㅎ has a softer and more polite tone than ㅋㅋㅋ and is more commonly in somewhat formal contexts.
• ㅎㅎ is more likely to be included "just for the sake of it" or to make the tone more friendly, whereas ㅋㅋㅋ is more likely to represent actual laughter.
• Unlike ㅋ, it is rather uncommon to repeat ㅎ more than three times.