This expression is useful for describing who a sentence is about. It's easiest to understand it with a few examples:

As a parent, I have to think about what's best for my kids, not just myself.

As the oldest member of the team, I feel like I'm held to a really high standard of performance.

As my friend, tell me the truth: do you think I'm making a huge mistake? 

You can see from the examples that "As ___" works for talking about yourself or for talking about other people.