8940 Motherboard - What is "Slot 1" - Picture included

It is PCIex1 slot.  It is used for low speed add in card.  #21 on diagram.  

a PCIe x1 slot is used to plug in low demanding PCIe expansion cards that do not have a very high throughput (transfer rate).  In 8940 it is a gen 3 PCIe.  The Gen 3 x1 slot has a throughput of about 1 GB/s.

The following are some of the cards that use the PCIe x1 slot.

1. Sound Card
2. SATA Expansion Card
3. USB Port Expansion
4. Network Cards
5. TV Tuner Cards
6. Video Capture Cards
7. PCIe Riser Cards

notice there is also a PCIex4 slot at bottom of mobo, #19.