Default field value
Default field value is the default value for new rows, i.g. it's the value for rows, created after this default value is set. Default field value is assigned at the moment a row is created and does not affect existing rows values.
Default field value is provided as a string value. Each field type has its own string format, for example for int values it's an int constant (10), for float value it's a float constant (10.1), for bool value it's 0 or 1. If you are not sure which string format to use for your particular field type - create a row with the value, you want to use as a default value and export it to Excel. Then copy Excel cell value and paste it to "Default value" column.