Seconds cloth diapers are cloth diapers with perfect function, but imperfect aesthetics. In short, they could be dirty, have a crooked tag, seams sewn with the wrong color thread, extra seams, etc. These are sold as a discount, and typically have no warranty.
They often have a mark placed on the tag to indicate that they are seconds quality. This prevents the warranty from being enacted and also for those in the know to re-sell them at a profit. After all, if they only paid 2/3 of the price new, why should you pay them 2/3 for them used?
A seconds quality will have a lower re-sale value since it has a lower initial cost. If it’s a pocket diaper, it doesn’t usually come with an insert. You only find these direct from manufacturers websites, and only a few times a year. Often the sales are not announced publicly until most if not all of the stock is gone, and often is only emailed or otherwise sent as a notification to private groups aligned with the manufacturer. If you know what diapers you like, and have the cash, seconds diapers are a great deal–if you can find them.