This is a standard stock/forex and now cryto trading term.

Some exchanges (Bitfinex for example) will allow you to trade on "margin" at a certain rate. For example you can buy 10 BTC but only put up the cash of 2 BTC (this would be a 5:1 margin). Depending on the exchange, you are borrowing the cash for the other 8 BTC at a specified % rate, (or on Bitfinex you can bid/ask borrowing terms on a SWAP exchange).

So why might you do this? Well if your borrowing rate is lower than the rate at which bitcoin price rises, then you buy with the borrowed money then sell when price is higher pay back the loan + interest and keep rest as profit.

You can do same in reverse; if you think bitcoin price is going to drop, then borrow bitcoins to sell now for cash, buy them back when price is lower and repay.

The term "leverage" is appropriate.. in the physical world you can use a lever to lift a heavy object with a fraction of the force otherwise required (think of teeter-totter where parent sits halfway up the bar and kid can lift them..).

So the advantage is it a way to multiply your profits! Disadvantage its a way to multiply your losses. If the price goes wrong way, then you get a margin call and have to add more cash / bitcoins to keep what you got, or they get sold off and can be left with dust...