keyword "in"  is used in Groovy to check whether element exists in Collection. 

Example List
String str = "this is me "
List list = str.tokenize(" ")
String word = "this"
assert  word in list == true
assert  "hello" in list == false

 Example with Set
Set set = ["this","is","me"]
String word = "this"
assert  word in set == true
assert  "hello" in set == false