adverb, adverb taking the 'to' particle
Describes a quiet, suppressed laugh, often shared among a small group or when trying not to be heard. Frequently used with 笑う.
See also: くすくす笑う (くすくすわらう)
友達とくすくす笑いながら話した。
I chatted with my friend while giggling quietly.
Someone is chuckling in the back of the classroom.