Yes, yes it is. http://icloud.com is Apple's website for all iCloud services, including Activation Lock.
If you have not done so follow the support article linked below to reset the iPhone back to factory condition so the next owner can set it up as their own.
What to do before selling or giving away your iPhone, iPad, or iPod touch - Apple Support
You can restore it from icloud.com if you wish, and remove Activation Lock from it as well.
Find My iPhone Activation Lock - Apple Support
iCloud: Activation Lock
Turn off Find My iPhone Activation Lock - Apple Support
Please read the articles above and understand that if you do not correctly remove Activation Lock, the next owner will not be able to use the iPhone you are selling them.