What do you understand by File modes in Unix?

File modes are basically permissions or access rights. The permissions that are given to the "user", "group" and "others" classes. It indicates to the system the types of files that each person can access, as well as the amount of access each one is allowed.

There are four main classes; each one with its own reference and set of permissions.

Reference	Class	Description
u	user	the owner of the file
g	group	users who are members of the file's group
o	others	users who are not the owner of the file or members of the group
a	all	all three of the above, is the same as ugo

Once assigned, the modes are not permanent. They can be changed. In traditional Unix permissions, modes can be changed with chmod. In Access Control Lists, modes can be changed with setfacl (for).