I don't understand the function of the option -f added to the tail command.I know that tail views the last part of a file. The manual says that-f outputs appended data as the file growsBut I don't really understand what that means. I know the meaning of each word in the previous quotation, but I don't understand the meaning of the whole sentence.
What does tail -f  do?
tail
It means tail -f command will wait for new strings in the file and show these strings dynamically. This command useful for observing log files .For example try, tail -f /var/log/messages.