While this might mean to make a shrill cry, or to laugh or talk in a shrill manner, or even the act of a large group of women in one room talking and laughing, but:

1. It is a word used in response to a silly or a 'not so funny' joke. One simply Says "CACKLE!" and walks away with a serious look on their face.

2. It may also be used in place of the redundant "Laugh Out Loud" acronym LOL.