What does this mean? ><

Development Tools IDE 1.x
1

Arduino: 1.6.6 (Windows 8.1), Board: "Arduino/Genuino Uno"

Sketch uses 1,174 bytes (3%) of program storage space. Maximum is 32,256 bytes.
Global variables use 9 bytes (0%) of dynamic memory, leaving 2,039 bytes for local variables. Maximum is 2,048 bytes.
avrdude: ser_open(): can't open device "\.\COM3": The system cannot find the file specified.

Problem uploading to board. See http://www.arduino.cc/en/Guide/Troubleshooting#upload for suggestions.

This report would have more information with
"Show verbose output during compilation"
enabled in File > Preferences.

2

Driver problem between PC and USB/Serial adapter on the Arduino. Try the Loopback Test thread.

3

May also want to upgrade to Win10.

4

Check tools -> ports - is COM3 selected? Is there any other option there?

Windows changes the port number under some circumstances, and that will confuse the IDE if the arduino is no longer on com3 when it previously was.