Electric bass strings with a gauge of 40 are considered Super Light. These strings are thinner and provide a lighter tension, making them easier to play for some players and producing brighter more melodic tones. The gauges used for bass strings can vary between manufacturers, so it's important to check the specifications for a specific set of strings before making a purchase, but you can't go wrong with Ernie Ball's Extra Slinky Super Lights, if you want to test the waters.