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What is mean by Per VLAN STP ? What is their role in STP?

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Cisco has their own

Cisco has their own implementation of STP.

Per VLAN means that each VLAN has its own STP instance.
So if you have for example trunk interface then it can have different roles in each VLAN.

Also it can provide you flexibility in design. Just imagine that you have only one STP instance and 2 VLANs. Then port blocked in one VLAN is blocked in all VLAN. It is good but you have used one port of switch which is not really used for traffic until something fails. So you have used resources but they are blocked and waiting for failure.

But now if each VLAN has its own STP instance then one port can be blocked in first VLAN but the same port is forwarding in second VLAN. So you can better use resources.

Everything depends on design and configuration.

But on other side, if you have too many VLANs, let say about 2000 then it means that you will have 2000 STP instances which consume hardware resources. So there is place to implement multiple STP where you can configure several STP instances and then assign VLANs to these instances (more VLANs use the same STP instance).

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An addendum to what Milos has described, MST is a standard, while PVST is Cisco specific.  It and RSTP also have features not found in original STP.  Cisco's PVST supports much of the same additional features, but you often have to enable them via configuration.

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