What is /usr/bin directory on a linux, unix operating system

user binary directory or /usr/bin Is used to store user executable applications.

/usr/bin is also used as a look up directory by the shell so when you type a command the operating system will fetch /usr/bin for matching binary files

As compared to /bin and /sbin , /usr/bin is a common  directory used to store user installed application files let's say text editors or webservers.