You use your investment as collateral for a loan, then you use that loan to buy more collateral.

Essentially in this context, say you have 1 Bitcoin on Coinbase.
You're telling Coinbase:

Hey, I have $10k worth of collateral on your platform; give me a loan.

So let's say Coinbase agrees and gives you a loan for 10k. Margin trading is when you take this loan and then use it to buy more Bitcoin. This would be a 2x trade, because your collateral is worth 10k and you bought 10k more on credit.

Unfortunately, let's say Bitcoin crashes 20% on you and your Bitcoin collateral is now only worth $8000. Coinbase might sell some of your Bitcoin because they're worried that you won't be able to pay back the loan they gave you.

However, if Bitcoin were to do something crazy like go x3... you just made a ton of money because your 2 Bitcoin is now worth $60,000. You can pay off the loan you took out and still have $50,000 leftover instead of the $30,000 it would have been had you not margin traded.