This library provides a Java API for accessing the DeCONZ REST and websocket APIs. DeCONZ is software which can talk to the RaspBee or ConBee hardware. These devices allow your Raspberry Pi, desktop computer, or laptop to control devices on a ZigBee network and respond to device events. This Java API lets you efficiently query and control all your ZigBee devices.
For example, you can control Philips Hue light bulbs and LED light strips without using the Philips Hue bridge. Why would you want to do this? With the Philips Hue bridge you must poll for events -- in other words, it won't tell you when something occurs, instead you must ask it, "has a button been pressed?". If you want to react to events quickly, you would need to ask the bridge many times per second. This is inefficient, wasteful (especially for large ZigBee networks), and still doesn't provide good response times.
With the DeCONZ websocket API, you get a notification the instant an event occurs. Not only is the response times as fast as possible, you aren't limited to the built-in rule actions. You can have your code do anything, allowing your switches and sensors to control non-Philips Hue devices, play music, open the garage door, etc.
Another advantage to DeCONZ events is that you can keep an up to date cache of the light states. When you need to know the brightness or color of a light, you don't need to wait for the light to be queried and reply with its state -- you already know the state because the last time its state changed, you received a notification. Knowing the device states without a delay allows your ZigBee automation to be smarter when a user has made manual changes. For example, if a user dims lights using a wall switch or cell phone app, your lighting control code can avoid overwriting those manual settings for some period of time.
DeCONZ provides a web interface for configuring your ZigBee devices. It can do everything the Philips Hue cell phone App can do (groups, scenes, rules, etc), but is a lot faster and has more functionality. For example, it can take actions on a schedule and can do a Touchlink scan to reset a device so it can join a different ZigBee network.