Well, from your question I assume you know what a line in
/etc/passwd is, so your question strikes me as a bit of odd. Unless, of course, you're going through some kind of test and don't really know your ways around a unix system and try to pass easily by cheating with us.
That's a line in
/etc/passwd that defines a user called
backdoor, which prefers the bash shell.
The bad news is (aside from the fact that someone utterly stupid or an author of some kind of admin test used the name
backdoor for this user) that this account uses user ID 0 and Group ID 0, and home
/root, and all three of those should be absolutely exclusive to
root, the super user.
Your system has been compromised.
You'll need to remove the system from the network, do a postmortem analysis, and set it back up from scratch, hopefully closing the vulnerability you found in your postmortem analysis that allowed them do this in the first place. (Leaving the system up and trying to "clean it up" is a losing game because who know what other rootkits or backdoors they have planted).