An ounce is a very, very small portion. The exact measurement of an ounce is that 1 ounce = 1/16th of a pound of weight.
You'll often hear ounces referred to when measuring out food items during baking, like butter or seasonings. Most US fluid ounces (also known as the imperial fluid ounce) are referred to in ounces as written on the cans or containers that it's in. This helps you to know the volume of liquid that is held inside.