What is `Permission denied`?

In Unix and Unix based operating systems, files are associated with certain permissions. They include read, write, and executable permissions.
Files cannot be executed unless they have the executable permission.

chmod command

chmod stands for change mode and as that name implies, the chmod command is used to change the mode (permissions) for files.

Syntax: chmod [reference][operator][mode] file…

Types of file permissions:

Read ( r ) = only read the contents of the file

Write ( w ) = only write to the file

Executable ( x ) = execute the file

After providing executable permissions to the file, it can be executed as ./<file_name>

what do i do if it says cannot open path of the current working directory: Permission denied
this happens when i use tree command