What does the 'At sign' mean in yaml config for azure pipeline

these are indeed version numbers. usually there are some changes between task behaviour\capabilities between different versions. task syntax might remain intact (although its not always the case).

I dont know what is the difference in this particular case, but given this is a copy files task, there probably is none.

What does the 'At sign' mean in yaml config for azure pipeline

Yes, it represents the task version. More precisely, the major version.

According the document Tasks:

In YAML, you specify the major version using @ in the task name. For example, to pin to version 2 of the task

Besides,

what is the difference between CopyFiles@1 and CopyFiles@2

I am afraid no one can completely list the exact difference between the two versions task. This is a general iterative process of software development. When we release the initial version, we will continue to repeat our products based on user feedback. When we decide to add a major version, we will release the next major version. This is the origin of V2.

And from the history of CopyFilesV2, we could to know the know the iterations and differences between different Minor or Patch versions.

So, the difference between CopyFiles@1 and CopyFiles@2 should be the fixes of multiple issues and the improvement of functionality.

Hope this helps.