When you created a Salesforce Sandbox, the user creating the new sandbox was kept active to be able to login in this new sandbox, but all the other ones were created inactive.
Now, since the last release, you can choose to keep active, all the users of a Public Group, by example an Admin / Tech Lead active group, or for release deployment user, that may be granted access on all sandboxes.
You are required to select a public group. That means you need to configure such Public Group to be able to go beyond this screen. As it is an one-off operation, that is something you can configure up front, and review when needed when resources leave or arrive.
As you can see on the screen, the group is mandatory. Otherwise, it will generate an error asking you to enter a valid group name
To read more on the subject :
- Salesforce help : Considerations about creating a sandbox
- Salesforce help : Sandbox refresh instructions
- Salesforce Help: Inactive User Freezing
- Salesforce Help: Freeze or Unfreeze User Accounts