Reactions are SBC's form of likes and an extension of Reputation. They were introduced to give more variety to the simple like system once used. Currently there are eight available: Happy, Sad, Funny, Confused, Thanks, Wow, God Himself and Hug. Users get a notification every time someone reacts to their post.
There also exists various reactions on the Discord, but for simplicity, this article will only cover what's on the forum.
The original react. Self-explanatory.
Happy features a picture of Plankton. Typically used for good news or general nice things happening.
Sad is a picture of Mr. Krabs playing the violin. Used for sad things happening or as a way to show disapproval.
Funny is a picture of Squidward laughing. It was one of the two new reacts added to replace Meh. It was originally a photo of Sandy laughing before Seahorse 15.
Confused is a picture of the famous surprised Patrick. This is the only react that does not increase your like count.
Thanks is a picture of SpongeBob saluting. Self-explanatory.
Wow is Reg's famous tattoo from the Salty Spitoon. It's the other of the two reacts that replaced Meh.
God Himself is a picture of the almighty Danny DeVito. He can be used as a "super-like" for content that is stupendously godly. The icon was originally Jackie Chan prior to July 2020.
Hug is the newest reaction, and was added on December 2, 2018. It is a picture of SpongeBob and Patrick hugging over a tub of ice cream, and is used to show a sort of love or togetherness.
Meh was a picture of Squidward. He was used to signify dislike, disapproval or indifference. After a fight broke out on the SBC's Top 50 Best Episodes Ever! 2018 Edition thread, it was removed. This react also inspired MDPP to attempt to file a restraining order against Homie.