A term used to act overly excited, often about something you could not care less about. This is because to most people, trees are not very interesting at all.
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!
"Today, I bought a soda."
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!"