What are pre market and post market sessions and orders?

Pre-open session:

The NSE pioneered the concept of the pre-open session to reduce the volatility of securities during the market's daily opening. Between 9:00 and 9:15 a.m., the NSE and BSE hold a pre-open session.

Orders are collected, modified, or cancelled during the first 8 minutes of the pre-market session (between 9:00 AM and 9:08 AM). Limit orders and market orders are both possible. The order collection window will close between 9:07 AM and 9:08 AM.

New orders cannot be placed after the collection window closes at 9.15 a.m. Orders are matched, and trades are completed. During the order collection period, you can only place orders on the equity segment. More information can be found here.

Post-closing session:

Post-market orders, like pre-market orders, are only permitted for equity trading. From 3:40 PM to 4:00 PM, the post-market or closing session is open.

People can place buy/sell orders in equity (delivery segment using the CNC product code) at the market price during this session, but keep in mind that even if you place a market order, it will be placed on the exchange at the closing price. So, if the closing price of Reliance at 3:30 PM is Rs. 800, you can place market orders to buy/sell Reliance at market price between 3:40 PM and 4:00 PM (will be taken at Rs. 800).

The post-market session is relatively quiet, but you can monitor stock movement by opening the Market watch window between 3:40 PM and 4:00 PM.