Yup, it is exactly what it sounds like. Just text an emoji to one of the numbers provided by Pornhub and they will send you free porn that corresponds to a category of that emoji.
As for easter eggs to discover. Here are some of them:
Chocolate Doughnut – Anal
Live Long Prosper Hand – DP
Corn – Cornhub
Poop – Simply returns the text “ramsey”. No clue what it means.
Banana – Striptease
Toilet – Fetish
German Flag – German
Unicorn – Tranny
Trash – link to RedTube