It can also be used to change the subject or to break an awkward silence.
In speech, it is pronounced in a loud monotone voice, hence the all-caps.
It originated from the map Survival Island in Minecraft. Here, you only start of with one tree, so once you manage to get several trees, you're like: OMG TREES!
Guy 1: "What did the chicken say to the cow?"
Girl "I don't know, what?"
Guy 1: "Nothing! Chickens can't talk!"
Guy 2: "OMG TREES!"