Poke is a feature led by Facebook to get attention of any user (Who is your Friend on Facebook). You can compare Poke with the Buzz option we had in Yahoo Chat, so we used to click on Buzz and then the other person's chat used to vibrate and make sounds to get their attention.
Facebook Poke leave a notification without any message just saying that person poked you and you can poke him back to say "yes I remember you".
Auto Poke Back Trick
Poke All For Chrome is an extension which gives you the possibility to poke back all your friends together with one click. it even has that automatic feature which you can select and every poke will be reverted back automatically. ( you can install this extension only in chrome. )
After installation just visit Facebook and click on this hand icon in your status bar now you will see all your pokes you can select Auto Poke or just click on the Poke All option.