Barrels of oil equivalent or BOE is a measure of energy; it's almost always used to discuss primary energy. It compares any amount of energy to how much energy is contained in a single barrel of oil. The volume of a barrel of oil is quite specific and equal to about 35 imperial gallons (42 US gallons, about 159 liters). A barrel of oil is also significantly heavy. Since average domestic crude oil weighs about 7.21 pounds per gallon, a barrel of oil weighs around 300 pounds or about 136 kilograms.
A barrel of oil is a lot of energy, so it is typically used when exploration and production companies are reporting the amount of reserves they may have. By giving a BOE figure, analysts, investors and management can assess the total amount of energy the firm has access to, providing an equivalent unit for different amounts of primary energy for natural gas, coal or even uranium.