Margin trading is the term used when trading forex with borrowed capital. That is how you open $10,000 or $ 100,000 worth positions with only $50 or $1000 in your trading account. You can conduct relatively large transactions, very quickly and cheaply, with a small amount of initial capital.
There is a minimum amount of currency that we have to buy in order to open a position in foreign currency trading market. In forex terminology we call this minimum amount, a "lot". When you go to the super market you cannot just buy a biscuit. You will have to buy a whole packet. It does not make any sense to buy 1 Yen.
That is why they come in lots.
Carefully read the following example to understand the concept behind this.