The messages tab is used to configure image and text shown in each slide.
The image shown in the slide is selected here. There are multiple options including:
The primary message shown in the slide. This is required.
The page where the user is taken when the slide is clicked. Enter either complete address (e.g., https://yoursite.com/page) or
Placeholders are pieces of text which change their content dynamically when the notification is shown. More about placeholders can be read here.