How to reset all the buffers of the particular instances?
How to reset a user buffer? Is there any tcode for reset?

Resetting buffers can significantly change the performance of the entire system for a long time. It should therefore only be used where there is a good reason to do so. 

Here is an overview of the buffers you're looking for:

/$SYNC This resets all buffers of the application server
/$CUA This resets the CUA buffer of the application server
/$TAB This resets the TABLE buffers of the application server
/$NAM This resets the nametab buffer of the application server
/$DYNP - the screen buffer of the application server 

On which path should I run that command?

You should run the command where you normally run the transaction. 

Is it different effects if I just restart our server? Because restarting our server also clearing the SAP buffer right?

Restarting the buffer also cleans up the buffers. Sometimes resetting the buffers may lead to skipping of number ranges in certain transactions. But its very rare. And when buffers are reset, some transactions may take more time for it to be executed as it has to fetch the data from database. (its the same case if you restart the server).

What is SQL Buffer?

When the user send a request, an application has to be run supporting that particular request.  This application(SQL appl.) is stored in the Shared SQL Buffer area in SGA .SQL buffer is shared by all the workprocess to run the process.

In the documentation of function module EXIT_/SAPAPO/SAPLATPQ_010 there is the following statement:
"The function module can be used to initialize the data in the user exit function group. To be able to work with the following user exits, the module can save the call parameters in a buffer area."
What is the buffer area?
How should I access that values from other function modules that belongs to the function group?

The buffer area is the temporary storage area you would have defined for capturing the application logs. The log data are initially collected in local memory, and are then written to the database. This procedure speeds up processing and reduces the number of database accesses. It is also possible to write log data to the database individually, to avoid losing the log records collected up to that point in the event of termination of the application.The logged data can be read in the database and displayed on the screen. It is also possible to read and to display the log data which is buffered in local memory with a log number.

You can capture the detailed information, either for the whole log or for each individual log msg in two ways:

• Text module with any number of parameters 
• User exit with any number of parameters 

In this way it is possible, for example, to save lists which can be displayed when the log is analyzed, with the help of the user exit and will ensure that you are not using values for other users.

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