In pictures: Taking apart Hewlett-Packard's Prolia...
The Systems Insight Display shows the current status of the DL380. Any component faults will result in an amber light.
The front LEDs are from left to right: UID, internal status, power, NIC1, NIC2. The UID light can be turned on remotely through iLO2 (Integrated Lights-Out), or by physically pressing the LED. This can be used to identify a server that needs attention. The internal status LED and the power LED shine green, amber, or red to indicate a normal, degraded or critical status, respectively. The NIC1 and NIC2 LED are activity lights for the matching network cards.
The front access ports can be used to plug a monitor and a USB device such as keyboard or a mouse, instead of a system administrator having to go around to the back.
In pictures: Taking apart Hewlett-Packard's Proliant DL380 G5 server