Finley from Shopify here, thank you for reaching out!
How are you finding the store set-up process so far?
Having an organized main menu is important so that customers can easily find the products they need on your website, so I'm glad to hear you've been working on this.
From what you've shared, it sounds like there could be two scenarios - either the Dresses button could be linked to an individual product or the collection you've linked the main menu item to only has one product in it.
To see what the Dresses menu item is currently linked to, you can navigate to your Shopify admin > Online Store > Navigation > Main menu > Dresses > Edit like so:
As shown above, when you click edit, you'll be able to see what the button is linked to and change it if need be by clicking the "x". If you've linked to a specific product, please change this to a collection instead.
If the Dresses menu item is linked to a collection already, then you'll need to navigate to the Collections section to add more products like so:
A clear navigation menu
A clear navigation menu is one of the points discussed in the Homepage design 101: What to include on the front page of your website article on our Shopify Blog. You may be interested in giving that one a read. It has some other helpful points that you might look at applying to your store as well, such as adding eye-catching imagery and social proof onto your website.
The blog hosts a myriad of other articles with e-commerce tips and inspiration for your business. Additionally, Shopify Compass has really helpful video courses to follow along with, including our course on How to Design Your Online Store with Zero Design Experience.
Should you have any questions about your main menu, please let me know!
All the best,