What's the jumper "enable 1.5GB PHY" on a Western Digital hard disk?

It's SATA II disk, right?

And if you put jumper on these pins HDDs will be SATA I.

It's there for if your motherboard doesn't support SATA II.

PHY stands for "physical layer". By jumpering pins 5 & 6 will cause the drive to go into a legacy 1.5 Gbit/s mode, rather than its default 3.0 Gbit/s mode. Check out the Wikipedia article on SATA for a bit more info.

By jumping the two pins, 5 and 6, you limit the transfer rate to 1.5 GB/s.

PHY is simply the physical layer in question, between the drive and cables/transfer.

Take a look at Wikipedia's page on SATA throughput.