If you want to add a new autoresponder email in your autoresponder series and you want the subscribers that already went through your autoresponder series to also receive the message, you need to select "Yes" in "Past Autoresponder" dropdown :
Here's how it works, let's say you created a autoresponder sequence of 10 autoresponder messages which will send over the next 10 days. Now a subscriber subscribes to your list and receives all the messages over the 10 days, but afterwards you want to add a autoresponder message to the sequence, this is where the Past Autoresponder option comes in.
If you select this option when creating a autoresponder message then if the subscriber has past the time that you set the added autoresponder message to send then the subscriber will receive the autoresponder message immediately. If the subscriber has not past the time set when the autoresponder should send out then the subscriber will receive the message at the set time. If selected "No" then only new subscribers that signed up after you added the autoreponder message will receive the newly added autoresponder message.