Also called: pivot tuple truncation, _bt_truncate
In plain English
When a B-tree splits, the boundary key copied up to the parent (a pivot tuple) only needs to be just distinct enough to separate the left page from the right. Suffix truncation (_bt_truncate() in nbtutils.c, PG12+) keeps only the leading column(s) needed to tell them apart and discards the trailing ones. The pivot becomes a router, not a copy of real data.
Why it matters
Smaller pivot tuples mean more entries fit on each internal page, which raises fan-out and lowers the tree’s height — so lookups touch fewer pages. This is why putting a short, selective column first in a composite index helps the index stay shallow and fast.