When a site gets compromised, the attackers will often leave some piece of malware behind to allow them access back to the site. Hackers want to leave a door open to retain control of the website and to reinfect it continuously. This type of malware is called a backdoor.
What Are Backdoors?
Backdoors are types of malware that allow for remote control of a compromised website by bypassing appropriate authentication methods.
Source: Scuri check