This product is purchased based on minimums and units.
The Min is set to 6 and the units is set to 3
Quantity Minimums and Units are designed to more or less force the customer to make purchases of a Minimum Quantity ... and if need be, in Units.
This product can only be purchased if you buy at least 6 ... and after that in units of 3 ... 9, 12, 15, 18, etc.
If you do not purchase it in the right Quantity, you will not be able to checkout.
When adding to the cart, the quantity box on the product_info page is "smart". It will adjust itself based on what is in the cart.
The Add to Cart buttons are also smart on New Products and Product Listing ... these also will adjust what is added to the cart.
For example: If there is 1 in the cart, the next use of Add to Cart will add 5 more to make the Minimum of 6. Add again and 3 more will be added to keep the Units correct.
Product Quantity Min/Unit Mix is for when a product has attributes.
If Mix is ON then a mix of attributes options may be used to make up the Quantity Minimum and Units. This means you can mix 1 blue, 3 silver and 2 green to get 6.
If the Mix is OFF then you may not mix 2 blue, 2 silver and 2 green to get 6.
This product has the Product Qty Min/Unit Mix set to ON