An adverb indicating a small, involuntary chuckle or giggle; an onomatopoeic/mimetic word often used in comics and conversation.
その一言にクスッと笑ってしまった。
That remark made me chuckle quietly.
漫画の登場人物がクスッとする場面が好きだ。
I like scenes where a manga character giggles quietly.
彼の照れた様子に周りがクスッと笑った。
Everyone around chuckled softly at his embarrassed look.
その冗談は真面目な場では言わない方がいいが、友達同士ならクスッと受けるだろう。
That joke isn't appropriate in a serious setting, but among friends it would get a quiet chuckle.