The original Spanning Tree Protocol standard only supported one STP instance / process for the entire switched network with all VLANs within the same topology. A disadvantage of having all the VLANs within the same process is it does not allow simple load balancing to occur by filtering the port for one VLAN and allowing it for another.
Cisco did work around this disadvantage by developing Per VLAN Spanning Tree Protocol or PVST. This allowed for a Spanning Tree Protocol process for each VLAN in the network in a switched topology.
With many VLANs on a switched network comes many spanning tree processes. The switch will need to process a BPDU for every single Per VLAN spanning tree instance that exists on the switches. Multiple Spanning Tree helps relieve this by allowing a single spanning tree process to cover a single or multiple VLANs with a shared switching topology or tree. This single shared process is commonly known as a MST Instance (MSTI)
Multiple MST Instances can exist on a single switch and cover a range of VLANs, for example MST Instance 1 may cover VLANs 1 to 3 and MST Instance 2 may cover VLAN 4. VLANs 1 to 3 will have a similar topology in terms of what ports are blocked but VLAN 4 will be able to differ from this if the costs are adjusted.
MST allows switches to be grouped logically into a single switch in a spanning tree topology, known as a Multiple Spanning Tree Region (MST Region). This provides compatibility with other switches that may not run Multiple Spanning Tree.