One of the things that makes Unix different from Windows are file ownership and permissions. A file's group is set to the primary group of the user that creates it initially. File permissions control who can do what to a file. They're divided into four sections. User permissions control what the user who owns a file can do to it.
Group permissions control what members of a group that owns a file can do to it.
World or other permissions control what everyone else can do to the file. For directories, it means you can cd into the directory. File modes are represented using letters and symbols familiar to humans. This is a bit more confusing, so I'll go into more detail here. The maximum base-10 value three bits can have is seven, which is the maximum value for a single digit in base 8.