Blocks were a form of digital currency available for use on Kongregate.com. They could be earned and then traded for Stickers that could then be used in chat conversations on the site.
While Blocks can still be earned and some Sticker Packs can still be purchased, the closing of chatrooms on Kongregate.com means that Stickers no longer have a place where they can be sent and currently offer no other functionality at this time.