Quote from spike500:
Delta is the change in the option premium if spot changes.
It is an approximate measure of the probability that the option will finish in the money. It measures the sensitivity of the premium ( or the value) of an option with respect to the underlying rate of the currency or stock at that moment.
If delta is close to 100 then the right to own the currency or stock is nearly as good as actually having them. So an option with a delta of 100 will behave in a similar way as the spot market.
Delta changes when prices change, so delta measures the probability at that moment, because if spot changes, the premium will change too and probably not in a 1 to 1 relation.
Delta is very useful for the concept of having a delta neutral position. Professional option hedgers usually use it to be protected from spot risk. Delta is used to hedge the CURRENT exposure. So when delta changes the hedge needs to be corrected to the new delta to become delta neutral again.