UPS batteries are a fundamental part of an Uninterruptible Power Supply.
The batteries store the energy from the mains power as chemical energy. When the mains power fails, the chemical energy is transferred into electrical energy and provides your uninterruptible power supply, thus allowing your critical operations to continue without disruption.
UPS batteries, like all components, have a limited design working life. Generally the batteries available have a design life of either 5 or 10 years. Because these timescales are based on ideal environmental conditions, which are rarely achieved in actual real world IT rooms, the actual life of a battery will vary quite considerably.
UPS batteries are the single most common component to fail on a UPS system. If they are not checked and tested regularly, they may fail considerably before the stated life expectancy. Just one faulty battery in a string of 32 could cause you to lose your critical load as there are is longer sufficient current and capacitance to support your loads when the mains power is offline.