As a basic guess, I would say it’s a sweetener.  Are you interested in something specific?
assitant
In my understanding, agave nectar is the same as pure honey. So it’s going to sweeten the drink the same way that honey would.  A few years ago, agave syrups were considered superior to honey, but the difference has been mostly academic.  Personally, I find agave nectar very tasty, and the difference is entirely worth it to me, but you should use what you like and you’ll find that there’s a variety of agave nectars to suit your taste.
assistant
What is the purpose of agave nectar in a cocktail?
user