In simple terms, Armitage is nothing but a GUI tool for performing and managing all the tasks that otherwise could have been performed through msfconsole.
Armitage helps visualize the targets, automatically recommends suitable exploits, and exposes the advanced post-exploitation features in the framework.
Remember, Armitage uses Metasploit at its backend; so in order to use Armitage, you need to have a running instance of Metasploit on your system. Armitage not only integrates with Metasploit but also with other tools such as NMAP for advanced port scanning and enumeration.
Armitage comes preinstalled on a default Kali Linux installation.