In a standard lot, it represents 100,000 units of currency.
Let's say you want to buy 100,000 units (one lot) of AUD/USD.
The Aussie against the US dollar is currently at $1.20
What happens is that you will receive 100,000 units of Australian Dollars!
In return, you will fork out (pay) $120,000 US Dollars.
Just to put things in perspective: