There are three Festivals in the Jewish calendar: Pesach, Succos and Shavuos. It is a mitzvah to go to Jerusalem for each of these Festivals and to offer there a sacrifice called a korban chagigah, a holiday offering. On Passover, the korban chagigah was offered in addition to the korban Pesach (the “Paschal lamb”). On the Seder plate, the zeroa (shankbone) represents the korban Pesach, while the roasted egg represents the korban chagigah.
The obligation to appear at the Temple in Jerusalem for the Festivals is stated three times in the Torah: here, in Exodus 34:23 and in Deut. 16:16. Accordingly, we are required to do three separate things: show up in Jerusalem (Mitzvah #489), offer the korban chagigah (here) and to rejoice on the Festivals (Mitzvah #488).
The korban chagigah was what was called a “peace offering.” It had no prescribed limit and it could offered from any appropriate animal. The person had to bring the animal to Jerusalem, or money with which to buy an animal upon arrival.
The reason underlying this mitzvah is that everything we have comes from God. When we celebrate the Festivals at His “house,” it would be inappropriate for us to arrive empty-handed. He doesn’t need anything from us; it’s just good manners.
This mitzvah was only incumbent upon men who were physically able to make the trip to Jerusalem, and only when the Temple was standing. It is the subject of the Talmudic tractate of Chagigah, see specifically page 10b. It is codified in the Mishneh Torah in Hilchos Chagigah. It is #52 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos.