Uninterruptible power supplies are essentially a battery backup for your computers and critical services. They connect to your mains supply and have internal battery packs that provide storage during a power outage.
In a line interactive smart ups when the ac input voltage is stable and there are no issues with the incoming power, the device will filter the power and provide a stable output voltage to your devices or services. This is a healthy status, where the battery pack is not discharged.
This is called a double conversion mode where the line voltage supplies power to a rectifier which converts the AC to DC supply. This DC power is then inverted back to an AC output voltage providing power for the connected services or device.