The top gray bar on the Android screen displays important information about your Android phone.
Along with the time, battery state, and network state icons, Android will also show icons in this bar when there is an update that may need your attention, for example a new text message, email, or voicemail. This bar, also called the windowshade because you can pull it down to cover your screen, is usually the quickest and easiest way to interact with those new and incoming events.
Here is a video from Google that demonstrates the windowshade feature:
Pull it down by touching the bar and then swiping your finger to the bottom of your screen … the bar will stretch and open up just like a window shade.
Newly installed applications will also leave a notice inside this windowshade, to give you the option of launching the application from there rather than pulling up the big menu and finding it.
Try observing your windowshade and interacting with it through the day. Android has done a smart thing by implementing this feature; rather than interrupting whatever you are currently doing when an incoming event, it will instead give a little beep, flash a notice of the new event in the gray bar, and then leave an icon there until you choose to interact with that event.
Here is a listing of the various possible notification icons you may see in the windowshade, and their meanings:
Battery is charging
Battery levels: full, 75%, 50%, 25%
Call is in progress
Call is on hold
New email received
New text message received
New SMS/MMS received
New voicemail received
Visual voicemail (Verizon app) activity
Phone currently muted
Alarm is set
Call forwarding enabled
3G service connected, service roaming, service not connected
GPS enabled, GPS sending/receiving data
WiFi connected, WiFi not connected but networks detected
App downloaded or updated
Bluetooth enabled, Bluetooth device connected
More notifications available
Sync error or PC connection debug mode enabled
Audio file playing
Upcoming calendar event
Uploading / downloading software
Airplane mode activated (cellular & WiFi disabled)