Twitter emojis are, basically, hashtags that are paired with images. These were seen recently and regularly during the Olympic Games in Rio de Janeiro, as well as throughout the national conventions. And this 2016-17 NFL season, Twitter-approved hashtags will be accompanied by corresponding team logos.
Some of the hashtags are simply the team nickname: #Patriots, #Broncos, and #Chargers. But many others play off famous team chants, like #GoPackGo (a regular chorus in Green Bay, Wisconsin), or team slogans. #KeepPounding, for example, the hashtag for the Panthers, was made famous by the late Sam Mills, a Carolina player and coach, who used the phrase before a 2004 playoff game against the #DallasCowboys.
This isn't the first time that Twitter has made hashtags trigger NFL emojis. They were piloted earlier this year, before the start of the playoffs—both for contending teams and the main event, Super Bowl 50 (#SB50).
It's a smart move for Twitter, and the NFL, as emojis will probably incentivize fans to use "official" hashtags when debating whose team is better, or discussing the latest highlight-reel play.