A checkbox field is no different. It has a value. When you check the box, it has the value you specify as “Checkbox value” here. When it is unchecked, it value is empty (or blank).Rather than saying “checked”, you can provide a more descriptive answer as to what the field value should be whenever it is checked. This is especially useful when you have multiple checkboxes linked together. In order to tell which one is checked, you need to specify a unique value for each of the checkboxes.In your example here, these checkboxes should have the following values:
1 hour
4 hours
8 hours
24 hours
2 days
3 days
So if you check the box that correspond to “24 hours”, the value of that field would be “24 hours”. If you leave these boxes unchecked, the value would be "" (empty).