What is vCPU in AWS
what is the difference between 0.25, 0.5, and 1 vCPU value?
It means that you're using a shared CPU where you will get only the amount you have selected as dedicated just like in virtual environments and virtual machines. So, 0.25 vCPU means that you will get dedicated 25% of 1 vCPU.
How does it differ by performance and number of threads?
AWS Fargate is a launch mode for AWS ECS which is based on AWS EC2, so whatever the vCPU and threads mean in AWS EC2, it is going to mean the same in AWS ECS.
A similar question has been asked before so you can check out the answer to that question here: What vCPUs in Fargate really mean?
Because AWS ECS is based on AWS EC2, you can convert these vCPU information into ECU information:
1 EC2 Compute Unit (ECU) is similar in power to a ~1.0-1.2 GHz 2007 Intel Xeon processor. (source)
1 EC2 vCPU corresponds on average to 4 ECU. (EC2 Tarification)
Hope it can help you to choose!