What does chmod 755 mean?

chmod 755 sets permissions using octal notation. Each digit represents a class: 7 = owner (4+2+1 = read+write+execute), 5 = group (4+0+1 = read+execute), 5 = others (4+0+1 = read+execute). So the owner can read, write, and execute; group and others can only read and execute — they cannot modify the file. This is the standard permission for executable files and directories. In symbolic form it is rwxr-xr-x. It is appropriate for web server directories, system scripts, and any file that should be executable by all but only writable by the owner.