This is the rate at which the biggest banks exchange currency between themselves and in very large amounts (millions at a time). This rate is usually what most online currency converters show as it's the most universally "correct", although it's not available outside of the largest financial institutions.
Banks and other financial institutions make their money by adding a margin, or mark-up, to the Interbank Rate or by charging fees as well.
In some cases, some companies might offer potential clients the Interbank Rate on an exchange they need to do. As this rate isn't a rate that regular consumers can access, it might be worth asking why a company is offering this rate as they wouldn't be making any profit on the exchange. Some companies offer the Interbank Rate to get custom and then recoup their losses at a later date by adding a margin once they've won the business.
The Interbank Rate is shown as one rate but can be made up of tiny differences between the amount a currency will sell for - known as the bid price - and the amount a currency can be bought for - the ask price.
The difference between the bid and ask is measured in pips. One pip is the smallest unit between any currency pair.