Position: opening or closing

Hi I use BMO InvestorLine and was just trying to learn more about options trading. While I don’t plan on doing any options trading anytime soon I figured it’d be a good thing to learn about. So anyways on Bmo InvestorLine, when you try to buy an option, apart from the type (call or put), expiry date, strike price, and number of contracts, you need to select a “position” which is either “opening” or “closing”. Can somebody explain what that means? Like if I was to buy a call option what would be the difference in a closing position vs opening position for it? Sorry if this is a dumb question. I tried googling it up but couldn’t really find a good answer. Thanks