system
A chat on subreddit /r/wallstreetbets
human
A call is an agreement to buy a 100 shares at a price, on a certain date, while a put is an agreement to sell 100 shares at a certain price on a certain date. You can exercise them anytime before the expiration as well. So if you buy a 3$ call that expires in a week for a stock worth 3.50 then that call has a value of at least .50x100=50$. 

Very simple, but if the price were to drop below the 3$ strike price of the call, and the call expires below, then it is worthless despite the stock itself still having a value. So also very dangerous. 

Calls and puts can be used to hedge risk in owning the underlying asset or used to gamble like many here on wsb do. I suggest being able to make successful momentum trades before getting into them, as its very easy to lose money.
gpt
So you're betting on the market going up or down? It doesn't need to hit the price it just has to move in the the direction you've bet on for you to make money? 

For example I want to bet on tsla going down so I open a put for say 50 bucks in a 3 month timeframe. If tsla hits 120 in 3 months I still win?

What's the best way in the uk? Robin Hood app isn't available in my region