He gave a IPv4 cidr block range of 10.0.0./16 but then set the subnets as /24, would you know why? I thought the cidr need to be the same when setting up a VPC? Furthermore, if he means I want to use 10.0.0.0 to 10.255.255.255 then it would be /8, if it was /16 then it would be 10.0.0.0 to 10.0.255.255, this is confusing.
You have the answer in the question. A subnet or subnetwork, is a network inside a network. Subnetting adds order, and security, and increased performance by splitting up traffic in larger networks.
Understanding subnetting is a required skill if you want to pass this exam.
I would look into the ACG course, Subnetting Fundamentals to learn this before continuing with this course.