Backdoor:PHP/Obfu allows unauthorized access to a computer system.
It is a remote access tool that allows a hacker to gain access to a compromised computer, typically though a TCP or UDP port.
Backdoors are usually standalone files that install themselves to the system after they are run. They then remain active in the memory listening on specific network ports for commands from a remote host.
The variants in this particular family are usually sent to the compromised server through a script that exploits an RFI (Remote File Inclusion) vulnerability on the compromised machine.