In your Paywall, it's required to configure the default plans and products.
When configuring your membership product, you'll be asked to create the Default Plan and Default Product. Each paywall has a default product and each product has a default plan.
Default Product
When a user signs up for your community for the first time, they'll default to a product within your Paywall. This can be any of the products that you're planning to offer. More times than not, you'll actually just put them on the free plan (and try to up-sell them) but it's possible to put them on one of your paid plans -- this could work if you're offering a free trial of some sort.
Default Plan
The default plan is used on the product listing page. When viewing a product on the product listing page, they'll see all of the available plans that they can select (daily, weekly, monthly, etc...) but one plan will be selected by default.
You can see that the MONTHLY plan is automatically selected in this view when deciding to select the plan. A user can opt to pick the annual plan if they'd like to - but by default, the monthly plan is presented to the user.
SIDE NOTE: Another, "The Office" reference for all of you Dwight fans ;)