On Linux/Unix machines many logs files generated on basis of process running on them. Usually every application and process as has some mechanism to log what running in it.
We can change level of logs, Like Critical, Error, Warning , info, Debug etc.
These logs are continually running on their logs files which help in case we need to know and understand what was happened in particular time regarding some process and application.
We can managed these in various ways in Linux/unix machines as per their requirements of application owners.

In some cases, we need to understand what running on application on real time which get logged in log file, but cat, less, vi like tools are allow continuous update on file.
So if we open any file in this way, it will not update any update on it for during this time. So here we have requirement of tool which could allow continuous update on open file.
For same in Linux/Unix machine we have tail command which has -f option that do same thing.

 -f, --follow[={name|descriptor}]
        output appended data as the file grows;

We can use this command like below

tail -f <Log-file-path>

Above command will only show 10 last line and further update will come below it
But in case we need to last 100 lines follow through update, we can use -n options to increase number of last line.

tail -n 100 -f <Log-file-path>

In Some Linux distributions, they also have alias tailf by-default which work same like tail -f

In Above mentioned ways, we can see any file in real time with all update and changes while opening file with tail command.