Optional human-readable description of this slave. This information is displayed on the project configuration screen.
When you have slaves that are different from others, it's often helpful to use this field to explain what is different. For example, entering "Windows slave" in this field would allow project owners to choose to always build on a Windows machine (for example, if they need some Windows-specific build tool.)
For EC2 slaves, this is also used to allow the same AMI to be used multiple times and separately manage the number of instances associated with each slave. The description forms part of the key (with the AMI id) to determine the number of active instances.