It means that you are telling pg that you don’t care about order, so it is free to optimize the query in whatever way it wants to. The order can change query to query depending on numerous external factors.

I’m not using pg itself as an example. I’m using a specific implementation of a “job queue” built with pg.

I’ve seen and you can search for and find many implementations of “job queues” using relational databases where job start order guarantees are traded away for throughput.