pepeJAM is an animated Twitch emote of Pepe the Frog with a pair of over-ear headphones on bouncing up and down to music. Like a lot of other types of emotes like this, it is used to show approval of music being played on stream.
The origin of this emote outside of Twitch is still unknown to this day, but it was first referred to on Reddit by user RoastCat23 as a way to show enjoying ironically bad music.
It then was used outside of Twitch to represent jamming to EDM.
On January 11th, 2018, the emote was added to the DiscordEmoji database by an unknown user. It wasn’t really used until October 11th, when a Change.org petition called for xQc to add the emote back to his stream.
Like many other emotes that were enabled on xQc’s chat, it gained crazy attention in a very short period of time. After that, it was pretty well known across the platform.
Is the chat or the streamer vibing to some good music? There’s a good chance you will see pepeJAM.