In-order is a method used to traverse a binary tree, where the nodes are visited in a specific sequence. The process involves visiting the left subtree first, then the current node, and finally the right subtree. This approach ensures that the nodes are processed in ascending order for binary search trees. This traversal technique is commonly used in computer science for various applications, such as sorting data or generating a sorted list from a binary tree. Understanding in-order traversal is essential for working with data structures like binary trees and implementing algorithms that require ordered data.