Receiving Warning Message: Cannot Modify Header Information

I’m receiving the follow warning message sporadically. It started after I uninstalled a plugin that was supposed to have installed my google analytics, but never communicated. Right before deactivating and uninstalling, I added my google analytics code into my functions.php file.

This is the message I am receiving:
Warning: Cannot modify header information – headers already sent by (output started at /home/fabuless/public_html/pinkheelspinktruck.com/wp-content/themes/lifestyle/functions.php:171) in /home/fabuless/public_html/pinkheelspinktruck.com/wp-includes/pluggable.php on line 876

Can anyone tell me what this means? I added my google analytics code to the very end of my functions.php file. It save the file correctly. And the warning message didn’t occur until after I was trying to remove the plugin mentioned above.
Website: pinkheelspinktruck.com