SAE 10w-40 Oil represents lubricants that have a viscosity grade of SAE (Society of Automotive Engineers) 10w-40. This grading system tells you the thickness of the oil and how it is affected by temperature.
In this case, the "40" represents the thickness of the oil at operating temperature. The higher the number, the thicker the oil is between 2 mating surfaces. The "10w" tells you how well it maintains that thickness in colder temperatures. The lower the number, the better the oil flows at cold temperatures.