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Brackets that represent a filled tick-box. Hence the writer is asserting that the statement following the brackets is true, or wants to highlight the presence of something or other.

{x} is the opposite of { }, an unchecked tick-box which means the statement is false or missing.

Multiple tick-boxes are often strung together, like a check-list of true and false statements.

(Note - curly brackets are used in the examples, since urbandictionary uses square brackets to denote links. However in real usage, square brackets should always be used.)
Quote: "I just really love bananaz. They are the best fruit by miles!!!1!"

{x} bananaz
{x} fruit experts itt
{ } OP has better things to do than posting this shiz

by Tickboxman August 12, 2010
X = Means Sabotage.
X has just pwned you!
by SabotageX June 21, 2005