What are those icons at the top of the screen?

The status bar at the top of the home screen contains icons that help you monitor your phone.

These are the icons from apps that came on your phone and the phone status icons.

CALLS
Active call.
Missed call.
Speakerphone on.
Microphone muted.
NETWORK
Connected to cellular/mobile network (full signal). Speed of your network connection is also shown. For example, or . Possible speeds, from slowest to fastest, are 1X, 2G, 3G, H, H+, 4G. Available speeds depend on your carrier and your location.
Poor connection to cellular/mobile network.
Connected to different cellular/mobile network (roaming).
Wi‑Fi network within range.
Connected to Wi‑Fi network.
Poor connection to Wi‑Fi network.
No SIM card.
Location services are in use.
Airplane mode on.
CONNECTIVITY
Bluetooth on.
Trusted device connected.
Your phone is a Wi‑Fi hotspot. Devices can connect to share your data connection.
Wireless display or adaptor connected.
Connected by USB cable.
USB tethering enabled.
SYNC AND UPDATES
App updates are available for download from Play Store.
App successfully installed from Play Store.
Email and calendar sync in progress.
Upload complete.
Download complete.
Google+ backup in progress.
Google+ backup complete.
SOUND
Vibrate.
Song playing.
DO NOT DISTURB
Do not disturb mode is set to Total silence. Your phone does not ring for incoming calls or notifications.
Do not disturb mode is set to Alarms only or Priority.
BATTERY
Battery fully charged.
Battery charging.
Battery low.
Phone is in Battery Saver mode.
ALARM AND CALENDAR
Alarm set.
Upcoming calendar event.
EMAIL AND MESSAGING
New Gmail message.
New text message.
New voicemail.
New message from Hangouts.
Important alert.
OTHER ICONS
More notifications available.
Screenshot is available.

Apps that you download might show other icons to alert you about their status.

Certain features, services, and applications are network or carrier dependent and may not be available in all areas.