automation testing strategy

Could anyone please explain, what is automation testing strategy?

the way you should do your testing. Mostly it depends heavily on the subject under test.  

Some tests are simple, which you can just write selenium code like  go to that URL and check it is really going there and stop.

But some are very much data driven and complex. there you need to figure out how to implement the test cases and frameworks.
This 'figuring out' is your test strategy.

Hope this helps!