You need to know everything obtainable about your customers. This list will get you started. You will need their age, sex, marital status, income levels, education levels, geographic locations, where they shop, if they go to church, if they have kids, ages of children, political affiliation, what they like about your company, what they don't like, why they shop with you, what they value about your products/services, what they like the most/least about products/services, and whether prospects are unaware of your brand or products/services. Customers want you to know certain things about them and if you do, they will remain loyal to you as you take time to know them.
The discovery meeting brings you and the marketing professional together to talk about different marketing services offered. They will be looking to understand your business better _ in its current state and your future vision. The marketing consultant will also explain the variety of marketing services offered so you will be better able to understand the final marketing plan when it is delivered.