It's an integrated circuit technology that utilizes switching to offer "more" PCIe lanes. Here is a decent Anandtech article from this time last year talking about it.
Warning: Spoiler! (Click to show)
With only sixteen PCIe 3.0 lanes available on a Z77 motherboard paired with an Ivy Bridge CPU, when we get to three or four-way GPU solutions these GPUs are itching to get more bandwidth. The Z77 specification limits us to three GPUs anyway, at x8/x4/x4. For some extra cost on the motherboard, we can add in a PLX PEX 8747 chip that effectively increases our PCIe 3.0 lane count, giving 32 PCIe 3.0 lanes overall.
From what I understand, while the mainstream non-PLX Z77 motherboards support a maximum of 16 PCIe 3.0 lanes (w/IVB cpus), X79 motherboards can support 48 PCIe lanes w/SB-E CPUs. I'm assuming that once IB-E is released these 48 lanes will be PCIe 3.0 also. Having only 16x PCIe 3.0 lanes limits the number of lanes for multi GPU setups, so manufacturers include the PLX PEX 8747 chip (or some other variant I guess), to emulate more lanes by switching. At least, this is how I understood it. My guess is this undoubtedly isn't as optimized as actually having more lanes, and probably presents some adverse effects, perhaps latency issues at the very least based on the diagram. This is why I said that if I knew that I wanted a multi-card setup (2+) from the get go, I would have purchased an X79 platform. I also like how they will be releasing an i7-4820k(?) which will bring unlocked affordable quad-cores to the X79 platform.
I did recently run into this motherboard, and was blown away at how they combined PLX with an X79 platform. Using an SB-E it allows for 64 PCIe lanes! They were able to include x16/x16/x16/x16 PCIe socket system WITH a built in LSI RAID controller. Technology moves so damn fast,
I hope that answers your question. And again, correct me if I'm wrong