Unix and Unix-based Operating Systems keep track of "File Permissions" for files and directories. These permissions typically state who is allowed to Read, Write, and Execute them. This methodology allows Unix to control the ability of users and programs to access files, as well as to prevent files which were not intended to be program files from being executed. Permissions are specified by a three digit number, such as 755 or 777.
In order for a Web Server to allow a program to run, it must have its execute permissions set. Additionally, any program which maintains data files must have those files be both readable and writable. In order to create new files in directories, the Web Server requires that the directory itself have read, write, and execute permissions set.
The Unix program chmod is usually responsible for changing File Permissions; however, most FTP Clients have the ability to change File Permissions. Try right-clicking on a file and looking for a command such as "Change File Permissions", "Change File Attributes", or "chmod". FTP programs sometimes allow you to specify attributes via checkboxes; however, the easiest way to specify Permissions is to enter the appropriate number in.