What is a VPC
We talk about just what an VPC is.
Let's cover how AWS is split into physical locations - this is pertinent to VPCs.
AWS has various regions across the globe. Each region is separate - switching between regions within the AWS console will make it look as tho you're in a different account.
VPC's are Virtual Private Clouds. They are, essentially, the private networks you put your resources (servers) into. Each VPC is separate, so instances in one VPC cannot speak to another instance in another VPC by default (you can do that if you set up VPC peering, but don't worry about that for now).
Each region is divided into multiple Availability Zones (AZs). These are separate physical buildings, relatively close to each other geographically.
Within a VPC, each subnet created is within a single AZ.
Since you create servers (and other compute instances) into specific Subnets, this is also how you spread resources across availability zones within a region - by choosing a subnet in which to create the resource.