wht is closure in groovy code example

Example 1: groovy3 closure

def listener = { e -> println "Clicked on $e.source" }      (1)
assert listener instanceof Closure
Closure callback = { println 'Done!' }                      (2)
Closure<Boolean> isTextFile = {
    File it -> it.name.endsWith('.txt')                     (3)
}

Example 2: groovy3 closure

{ [closureParameters -> ] statements }