That is ignore. So if you have a checkbox and set the checkbox value to "yes"... then when someone checks the box it gets set to "yes". If they uncheck the box then no value is submitted. If it is set to "ignore", then the database "yes" value remains.
The "ignore" option is really there for the password or hashed field values. You can't or wouldn't want to pre-fill the password field for security reasons, so you want to leave the old password when the field is left blank, but update it if someone enters a value.
Usually all other fields use "blank" or "null" to allow users to update and actually put in a blank value. You could also update the default to be a string like "No", so that if the value is blank (like an unchecked box) the string "No" is placed in that field.
