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.)

Got the covid-19 vaccine

X = Means Sabotage.