The Full form of ROFL is Rolling On Floor Laughing. ROFL is very popular internet slang nowadays between youngsters and commonly used on social networking sites like Instagram, Facebook etc., to indicate a bursting laugh on a funny situation. Generally, it is used in chatting between friends.
ROFL is only chat slang and nobody will find its meaning in any dictionaries. There are some other internet slang also which can be used instead of ROFL. They have almost the similar meaning. These internet slangs are: LMAO, LOL etc. These words are informal and can be used only in chatting or messaging on social networking sites.