TL;DR version: Skip to In a nutshell.
This article assumes you understand the notiation for file system permissions, and have a basic grasp on the topic. If not, there is a nice starter article on the Ubuntu community wiki:
By default, when you create a file in Ubuntu as a standard user (either in Nautilus or with
touch), it gets the permission of
644. If you mark that file as executable, as expected, it gets the permission
What may be surprising is that directories also have a default permission of
755. Even though it seems like directories aren’t the type of thing that would be “executed”, this flag is actually very important to Linux!