Command chmod changes the mode of files. The mode of a file controls the access permissions associated with that file. The read permission means that the user is able to look at the contents of that file. The write permission allows the user to modify the file. The execute permission means that the user is able to execute the file.
Directories have a slightly different behavior.
The read permission allows the user to to view the contents of the directory with the ls command. The write permission enables the user to create, delete, and modify files from the directory. The execute permission allows the user to change to the directory with the cd command. Verbosely describes changed permissions. Recursively changes permissions of directories and their contents.