The origonal meaning was to perform a feat that impresses the members of the Ultimate Frisbee team the Thundering Herd.
Now can also mean any hardcore or impresive move that captures the spirit of the Thundering Herd team.
Such as performing an optimistic huck and it being caught.
Creating a new peice of Herdchadise.
Partying especially hard.
If someone was to turn up to an Ultimate Frisbee game with little sleep, vomit at the side of the pitch, and then to continue and run around for an hour, A Herder may say, "that is Herdcore"