The US Products Database is designed to allow users to search for products that ship from an originating country that is close to your customer. That means that it is expected to arrive quicker.
In short, it simply means that AliExpress suppliers keep inventory on hand in the specified country. Also, this feature isn't just limited to the U.S. - there are "Ships from" options for the following countries: Spain, France, Italy, Germany, Poland, Czech Republic, Turkey, and China!
It's equipped with many options to filter and sort the database options.
Shipping times within the U.S. Products database are established by the supplier site. These sites provide their projected delivery times on the product page at AliExpress. As you conduct a search, you should notice that all suppliers have varying shipping times.
If you are shipping to U.S. based customers, it's common to see shipping from postal sites like FedEx, UPS, or USPS. Some suppliers have carriers like this that ship within 5 - 7 days or most commonly between 2 - 11 days.