When you're uploading an ipa or an apk in your store, you're being asked to add information about your app in the "Store page" :
This is important as it will help your user understand the purpose of your app! Add a few screenshots, a nice banner... and that's it! In just a few minuts, you've made your app much more attractive!
You can add :
a banner and introduction phrase
changelog (useful if you want to inform testers about changes in this version)
For instance, here's an app which has a banner and screenshots in the store:
And a description: