This returns you to your home directory. This moves you up one level in the directory hierarchy. This moves you to the subdirectory work within the current directory. This moves you to a subdirectory mystuff within your home directory. This moves you to a subdirectory stuff in Joe's home directory.
Numeric form is described here.
The highest possible value is 777 - which means everyone can read, write and execute the file. The owner can read, write and execute notes. The files specified group and all other users may only read the file. If no directory is specified, the contents of the current directory are listed. Lists the contents of the current directory in long form including hidden files.