Five months ago, I wrote the following as a response to a Redditor who asked how accept filters worked in FreeBSD, and wanted to have it explained like they were five years old. I’m posting it here, because it’s a recurring question, and I’d like it somewhere easy to find. Original thread.

Without accept filters: Imagine if someone were to send you a message by letter. They’d send one sentence the first day, the second sentence the second day, and so forth. You’d go check that mail box every day, because the ‘new mail’ flag was up. You piece the sentences together, and after a number of days you have the full message.

With accept filters: Imagine the above example, but the mail box scanned the contents of your letters, and wouldn’t raise the ‘new mail’ flag until there are enough sentences to form a full paragraph (request). You’d spend less time checking the mail box, and you’d still process the message at the earliest possible time.

The advantage to this is more noticeable when you have to check many mail boxes at the same time, and can skip the ones which don’t have a full paragraph yet.