To understand the LARProll, you must first become familiar with the duckroll and rickroll. A duckroll was when someone gave you a link, telling you that it was some really super-awesome thing, but it turned out to be a picture of a duck on wheels. The duckroll is now extinct, and has been replaced by the rickroll, which is when someone gives you a link saying it's really super-awesome but it's really a music video for Rick Astley's hit song "Never Gonna Give You Up".
Now, you must also become familiar with LARP, Live Action Role Playing. This is when people dress up in costumes to act out scenes from what you might see in some fantasy game or something. (If you search for LARP on YouTube, you'll get some good examples. "LIGHTNING BOLT!!")
A LARProll is starting to become popular on the discussion forums for the MMORPG The Lord of the Rings Online (it may be on other MMO forums too, but I wouldn't know). It is when people post a link saying that it is really cool in-game footage but turns out to be a LARP video.
fraps of last night's TR massacre! http://www.youtube.com/watch?v=j_ekugPKqFw
runekeeper in-game video http://www.youtube.com/watch?v=tB8VMNymEPA
These were both LARProlls that I have come across.