What is the meaning of the blue PCIe slot on the Dell T7910

2 is a slot that can interface with SATA , PCI Express

A card that bumps up to the physically larger x4 or x8 slot, like a USB expansion card, can transfer data four or eight times faster---and

whats a slot There are two full PCIe X16 slots The blue is the primary one

whatsapp slot 16x is physical slot size, that is capable of wiring up to 16 PCIe lanes, but on your motherboard that slot has only 4 lanes connected to it