You are prompted for an administrator password when you attempt to create or modify a file or folder for which your user account does not have permission to modify. If this is happening within your home directory, it likely means that permissions are mis-set. You could fix this by running the program Disk Utility, selecting your startup drive, and clicking on the button reading Repair Disk Permissions.
If this is happening in a system folder, this is by design, to make it more difficult to break something. While it can be overridden (the simplest way would be at the terminal to type
chmod a+w -R /, but this is highly unadvised) through changing permissions, I would suggest that this is a Bad Idea.