Sometimes you want most of a collection to sort automatically, but a few hero products to stay exactly where you put them. That's what locking (pinning) positions is for.
Lock a product in place
In the collection's product list, move a product to the position you want and lock it. Locked products are held at their saved position every time the collection sorts — on both scheduled auto-sorts and manual Sort Now runs. Everything else sorts around the locked items by your chosen sort type. See Manually Change Product's Positions.
Pin to the top while still auto-sorting
This is the most common use: lock one or two products to positions 1 and 2, then let an automatic sort type arrange the rest. The pinned products always stay on top, and the remaining products keep re-ranking on schedule.
Locking and out-of-stock push-down
If you also push out-of-stock products to the bottom, note there is an option that lets push-down apply even to locked products when they sell out. If you want a locked product to keep its position regardless of stock, leave that option off. See Push Down/Up Products Options.
Good to know
If every product in a collection is locked, there's nothing left to sort, so the order won't change.
Locks are per collection — the same product can be locked in one collection and sort freely in another.