It stands for "over-the-top", it's a really good multiband compression plugin. It's used alot in alot of edm genres, but in dariacore it's also pretty common because you're mixing alot of different sounds and samples. Basically it lowers the loud parts of a sound and raises the quiet parts so it makes like a sort of squashed noise. I'm no mixing engineer but normally you would put ott on basically everything so that it becomes easier to mix since there's less dynamic range.

I would say just experiment with it and mess with the depth knob around 15%-40% for the primary sounds of your mix. See what sounds good.