Wiktionary
watch it
vb. (context idiomatic English) To be careful or cautious; to pay attention to what one is doing (usually imperative).
Usage examples of "watch it".
Ray used to watch it and wonder, like the tree falling in the deserted forest, whether Simon bothered to light up if there was no- one there to see it.
And none of the watch it was dedicated to would have believed it for a minute.