What does "buffer" mean?

what does it mean when placing an order someone says use it with buffer?

Hi. If you went long/short on futures contracts and short on options right, the Span+Exposure will be blocked in your account.

Span margin is collected at the time of order placement and revised throughout the day. Brokers will receive the Span files from the exchange and the margin will be blocked as per the Span files. You can check the Span files from this NSE link.

Example: Let’s say, you have Rs.1,00,000 cash balance in your account, and you are shorting a future contract where the Span+Exposure requirement is Rs.99,000. In this case, your available margin will be Rs.1000. Now after the import of new Span files, let’s say the margin requirement has increased to Rs.1,10,000. Now as you didn’t have a balance in your account, your account will be in a debit/negative balance of Rs.10,000

To avoid this, you can have some extra cash/margin, which refers to as ‘Buffer margin’. Not only in F&O segment, even in cash segment it’s good to have some buffer margin.