It means only that technically you could use such an adapter like on this picture and use the available PCIe lanes to connect a standard PCIe card, which could be SATA controller card, 1G or 10Gb Network card, whatever, is just a matter of available linux driver.

That's said, keep in mind that the RK3399 SoC only has 2 PCIe lanes, and although they are stated to be version PCIe v2, their speed is still limited to 250MB/s per lane (same speed as PCIe v1), a question of power consumption I think.
The maximum you could theoretically get out of it, is a little bit less than 500Mo/s (4Gbit/s).