The internet is full of acronyms, and ROFL is an interesting one with a few different variations, including ROTF and ROTFLMAO.
Acronyms are abbreviations of words used to cut down on the time it takes to type. Acronyms are often typed and easier to understand when typed in capital letters.
There's no shame in seeing an acronym you don't recognize, and it's happened to everybody (or it eventually will).
ROFL means “Rolling On Floor Laughing”
ROFL is used anywhere you can respond to anything you find funny, whether it be a chat room, messaging, email, or text message.
ROFL is another way of saying LOL but indicating you find it funnier than the standard LOL.
And then, we have variations on ROFL, including whether you use T (The) or not. Some might say ROTF (Rolling On The Floor), ROTFL (Rolling On The Floor Laughing), or ROTFLMAO (Rolling On The Floor Laughing My Ass Off). You might also see LMAO, which makes it funny, just not so funny that you're rolling on the floor.
Don't worry; the odds are no one is actually on the floor.
For example: I knew a woman who owned a taser. Man, was she stunning! ROFLMA
Like anything that has evolved with the internet, there used to be generally considered right or wrong uses of ROFL, but you can use any of the examples above, and anyone who reads it will know what you mean.