Metasploitable is a Linux-based virtual machine. It is intentionally vulnerable and can be used to be exploited legally. Metasploitable Project is created and maintained by Rapid7 Community (Metasploit-FrameWork Community). In simple terms, Metasploitable is a Linux-based operating system, specifically designed for practicing penetration testing skills, network security skills. It can be accessed online, or installed on a virtual machine, at which point you will need to download it by clicking on this link.
I will use it to make scan demonstrations with different tools in some following articles. It is of course obvious that to be able to use the different tools towards the vulnerable Metasploitable machine, this one will have to be booted!