YOLO stands for 'you only live once'. People say it when they do something bold, silly, expensive, or a bit risky, and they want a reason — or an excuse. You buy an air ticket you cannot really afford, and you post the photo with 'YOLO'. You order the huge dessert, you dye your hair blue, you quit a safe job: YOLO. The idea behind it is old and serious — life is short, so use it — but the modern word is light and often a joke about yourself. It came out of internet and rap culture around 2011, and for a couple of years it was everywhere online. That is important for you: it now sounds a little dated, a bit 'my older cousin in 2013'. Young people today often use it half-mocking, the way you might use an old slogan on purpose. It is very informal — fine in a message to a friend, wrong in a school essay or a work email. Grammatically it is loose. Most often it stands alone at the end, like a stamp: 'Booked the flight. YOLO.' It can work as an adjective before a noun ('that was a very YOLO decision') and some people even use it as a verb ('I YOLO'd and bought it'), though that is playful. You can write it YOLO or yolo; capitals are more common.