A country selects a range, or "band", of values at which to set their currency, and returns to a fixed exchange rate if the value of their currency shifts outside this band. This allows for some revaluation, but tends to stabilize the currency's value within the band. In this sense, it is a compromise between a fixed (or "pegged") exchange rate and a floating exchange rate.
For example, the exchange rate of the renminbi of the mainland of the People's Republic of China has recently been based upon a currency band; the European Economic Community's "snake in the tunnel" was a similar concept that failed, but ultimately led to the establishment of the European Exchange Rate Mechanism (ERM) and ultimately the Euro.
A currency system that establishes a trading range that a currency's exchange rate can float between. A currency band represents the price floor and ceiling within which the price of a given currency can trade, and is like a hybrid of a fixed exchange rate and a floating exchange rate. The currency band restricts how much the price can move relative to a reference currency or currencies.