Can you explain the significance of the | symbol in TypeScript?

My journey with TypeScript is just beginning, and I recently encountered the symbol | while working on a problem in LeetCode using Typescript. I believe it has something to do with defining variable types. Can anyone provide more insight into this?

/**
 * Definition for a binary tree node.
 * class TreeNode {
 *     val: number
 *     left: TreeNode | null
 *     right: TreeNode | null
 *     constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
 *         this.val = (val===undefined ? 0 : val)
 *         this.left = (left===undefined ? null : left)
 *         this.right = (right===undefined ? null : right)
 *     }
 * }
 */
typescript

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