How long a device can operate on a single battery charge before needing to be plugged in again. Like the fuel tank capacity of a car - bigger means longer usage.
A smartphone with good battery life might last a full day of normal use, while a laptop might run for 8-10 hours.
Battery life is a device hardware characteristic, not a cloud service. Cloud providers don’t offer a direct equivalent, but cloud usage patterns (network activity, CPU load, screen-on time) can indirectly affect a device’s battery drain when using cloud apps.