What do you talk about with wedding planner?

That really depends on how involved she is, specifically with the other vendors. To give you kind of an idea, here is what my planner has done for us:

Recommend vendors, list vendors we need, review contracts, is point person of contact for any vendor questions, attends all meetings with vendors to ensure we are getting our vision, reminds us of hard and soft deadlines (ie, when to make payments, when to book alterations, when to schedule walkthroughs, etc), Listed things we should provide (like bathroom baskets/card box, etc), came up with our schedule, figured out the load in/break down/load out for all vendors, collected COIs, arranging for our things to get back to us after the wedding, suggesting who needs tipped/how much, came up with table layout, arranging flower drop off to getting ready locations, point person day of for any and all questions, supervising set up and flow of event and breakdown, attending to me and FH during reception to make sure we have everything we need...

Honestly I'm sure there is more too! If they are included with the venue they won't be handling a lot of this stuff, but hopefully this list is helpful in some way!