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The command 'chmod 755 filename' is used to set file permissions in a Linux environment. The 'chmod' command stands for "change mode," and it is specifically designed to define who can read, write, or execute a file.

In the case of '755', the numbers provide a clear breakdown of permissions for three different categories of users: the owner of the file, the group associated with the file, and all other users. Here’s how the permissions break down with the '755' settings:

- The first digit '7' represents the owner's permissions: it gives the owner read (4), write (2), and execute (1) permissions, totaling 7.

- The second digit '5' represents the group’s permissions: it grants read (4) and execute (1) permissions, totaling 5. The group does not have write permissions.

- The third digit '5' represents permissions for others: similarly, they also receive read (4) and execute (1) permissions, totaling 5. Again, others do not have write permissions.

Thus, using 'chmod 755 filename', the file permissions are configured to allow the owner full control while the group and other users can only read and execute the file.