This is a standard option in editable fields (all types but computed and attachment). You can automatically fill a field with a specific value every time the form is loaded. To find this option go to:
Open the field settings
Standard options
Default value
Creating vs editing records
This default value feature is commonly used when creating records. But this function applies when creating OR editing records. However, the default values will only be auto-filled if the field is empty. It will not override an existing value. It only works when a field has no value.
Default value vs prefill via URL
This feature works similarly to the prefill via URL function. It could be used as an easier alternative and a simpler way to achieve the same result. But there are some differences:
Hidden fields can still be edited when prefilled because URLs are manipulatable. Setting a default value prevents the end-user from changing it.
Default values also allow prefilling nested forms.
Prefilling via URL can override an existing value.