Additional "in ...." keyword after loop/if-statement

I can't describe the syntax I'm trying to understand too well because I can't seem to find it now that I'm looking for it specifically, but in some codes I stumbled upon a syntax like:

for .... { in something

someCode

}

I'm not sure either if was in a for/while-loop or an if-statement but I can't understand this "in...."-syntax after the opening curly brace.

I hope this information was sufficient enough.

Greetings and thanks in advance <3