Also called: page high key, upper bound key
In plain English
The high key is an upper-bound value stored as the first item on a B-tree page (every page except the rightmost on its level). It promises that every key on the page is less than or equal to it. A search compares its target against the high key to decide whether the key it wants might actually live on the right sibling instead.
Why it matters
The high key is what lets PostgreSQL’s B-tree be concurrent: if a split moved your key to the right after you read the parent’s pointer, the high key tells you so, and you follow the right-link instead of restarting. No high key, no safe “move right”.