In-order is a method used to traverse binary trees, 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 a non-decreasing 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. It is particularly useful when working with data structures like binary trees and binary search trees.