Literally "trout" in Spanish, it is also Spanish slang for "alert", or "vigilant". It was a combined with the word "Salvadorian" to form "Salvatrucha", as in Mara Salvatrucha, or the street gang MS13.
It is used as a warning, for example, similar to "heads-up".
A plate of brown consists of foods that have little to no nutritional value, and are typically deep fried. A true plate of brown will also be accompanied by a dark soda.