When creating a field in a GoCanvas App, you have the option to make it Required, meaning the person filling out the Submission won't be able to proceed to the next screen without providing the required field with an entry.
Required fields are marked with a red asterisk (*) to indicate that they are required to the user.
In the App Builder, you will find this setting under Field Settings:
Here's what it looks like on mobile, and the message the user receives when attempting to proceed to the next screen without filling out a required field.
Required fields are useful for capturing critically important information on a Submission.