Also called: right sibling pointer, btpo_next, B-link pointer
In plain English
Each B-tree page stores a pointer to its right sibling at the same level (the btpo_next field). These pointers chain every page on a level into a left-to-right list. A search that discovers its key has moved (via the high key) follows the right-link to find it, rather than going back up to the root.
Why it matters
The right-link is the heart of the Lehman-Yao “B-link” design. Because a split links the new page into the sibling chain before it updates the parent, readers can always reach a freshly split page through the right-link — which is why index reads almost never block on concurrent inserts.