Let's say we are inserting a new row in DB table. Is it possible to create a

Where are you inserting the record from? Is it from an external system?
There are two options.
(1) The ideal option is for the external system to send a WS call into Appian when a record is persisted. You can have it in the application level or in the DB level as a trigger that invokes a Stored procedure that makes a WS call. Most of today's db allow this to happen.
(2) To build a listener process in Appian that monitors change to DB and triggers the process when one happens.

Can you please brief on how to build a listener process in Appian that monitors change to DB and triggers the process when one happens? Is there any document which provide steps to do the same?

One option for the listener process is to launch a process model periodically based on a timer. The first node will be to query this database table for any new rows (which could be denoted by a Boolean flag) and launch a process for each new row. After the rows have had their processes generated you can update the Boolean flag in the table so the next time the poller runs it will only create processes for new table rows. The frequency at which the process runs can be determined in the timer node.

Thanks. What is mentioned here as "poller"?

Sorry for the confusion. Poller refers to the process which will periodically run to check your database for the new rows.

A poller is a generic term used to a design pattern that executes at a regular interval of time and does a unit of work. Think of it as a cron job. What Mike is saying here is that you will have a process where the start event will be configured as a Timer event (you can specify the duration) and it fires on a regular basis.
I would suggest that you ensure that the process is immediately cleared from memory in order to avoid data being retained in Appian for no reason. This will keep your memory lean.