Inventory is a primary business metric in Universal Paperclips. It appears only in Stage 1.
Summary
"Unsold Inventory" is the number of clips you have made which exceed demand. It's useful to have it high enough that you can see whether or not the number is rising or falling, and adjust accordingly. When your Price Per Clip is too high, the stock will rise, and when it is low, the stock will fall. An increase in the number of autoclippers or MegaClippers can also increase your inventory.
It is sometimes helpful to allow the numbers to climb while you are attending to other matters and then have a "sale" to bring down the inventory and raise money in a hurry. It's not like the clips have an expiration date.