In English, "LMAO" is an acronym for the phrase "Laughing My Ass Off." It is used to express enjoyment, generally during a conversation over a form of messenger, such as text messaging or instant messaging.
There are several acronyms commonly used in messaging that have the same or similar meaning to LMAO. ROFL is an acronym that stands for "Rolling On the Floor Laughing" and can be used interchangeably with LMAO. "Laughing Out Loud," or LOL, is used in the same way; however, LOL is commonly used to begin or end sentences or phrases as a tone marker, indicating light-heartedness but not literal laughter.