[x]

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.)