The Zou Arc, also known as the Zunesha Arc, features the Straw Hat Pirates (sans Sanji), Trafalgar Law, Kin'emon, and introduces new characters from the Mink Tribe like Wanda, Carrot, and Pedro.
The primary reason for this arc is that the Straw Hats head to Zou in order to link up with their companions and further plot against Kaido. Zou itself is a location within the One Piece world. It is an island on top of a giant, millennium-old elephant known as Zunesha that wanders the New World.
The Zou Arc opens with the Straw Hats and Law accompanied by a samurai climbing up the elephant's leg to reach the Mokomo Dukedom—the homeland of the Mink Tribe. The whole city is in ruins upon their arrival, setting forth a mystery throughout the arc. The Minks, a race of humanoid animals natively possessing Electro, welcome the pirates and tell them that their homeland was raided not long ago by Jack of the Kaido Group.
It also brings several key revelations throughout the story. The Minks explain their prolonged cooperation with the Kozuki clan of Wano Country by introducing something called "Road Poneglyphs," which basically helps them find One Piece. This links directly to the general plot of the series, setting up future story arcs. The arc goes further to explore Zou's past and the mysterious nature of Zunesha, the giant elephant.
One of the central conflicts of this arc would be the search for Raizo, a Wano ninja whom Jack was looking for when he attacked Zou. One Piece has recurring themes of honor and alliance, reflected in the dogged loyalty of the Minks' protection for Raizo despite their home being destroyed.
The real bombshell of the arc, of course, is the revelation of the Road Poneglyphs: ancient stones that lead to the final island where One Piece is located, when taken together. It's an enormous development in the real plot of the series and gives a clear goal for the future adventures of the Straw Hats.
Character development is the most visible characteristic for the arc. It really is here that the character development for the Straw Hats and their allies is the best. The formation of the Ninja-Pirate-Mink-Samurai Alliance marks the central moment of bringing people from different factions together against Kaido and Wano's shogun. It also sets up the separation of the Straw Hat crew: half the crew prepares to head toward Whole Cake Island to deal with Big Mom, and the other half plans to go off to Wano Country.
The Zou Arc comes to a close with the Straw Hats gaining powerful allies in the Minks and key information on the Road Poneglyphs. Thereafter, the plans of the alliance are set in motion: Luffy, along with half his crew, goes to rescue Sanji from Big Mom's territory, while the others train for the fight in Wano. It is the end of the arc, foreshadowing more trials and new pieces of information about mysteries around the world.
This arc is crucial to the storyline of One Piece in many ways. First, it adds another tribe of people to the already varied array of races existing in the One Piece world: the Mink Tribe. Second, it serves to enlighten a clear path toward the ultimate goal of finding the One Piece with the Road Poneglyphs. Finally, the arc acts as a bridge from the Dressrosa Saga and into the Whole Cake Island Saga, setting several storylines up for future arcs.