A wireframe represents the skeleton/structure of your app. It describe the functionality of the product and the relations between views (e.g. what happens when clicking the buttons). It presents the content/features and where these can be found in the app. The product's design is showcased by mockups, which represent the next stage after wireframes.

For this milestone you are only required to create wireframes for your product, but if you want to go for mockups, please feel free.

Check out Balsamiq (just a suggestion, you are free to use any tool).

If your product is not a web or mobile app, figure out how you can showcase its functionality.

If you don't know what a landing page is, you can find more information here.