For some fields, you may want to set default values for when a new record is created. For example, if you want to set a flag/boolean field to true or false when a new record is created. An easy way to do this is through a migration.
If you are just creating the field, you can do:
add_column :table_name, :field_name, :field_type, :default => 0
add_column :table_name, :field_name, :field_type, :default => true
To edit an existing field:
change_column :table_name, :field_name, :boolean, :default => true
You can place text in fields to pre-populate them or to save space by putting the label in the field with simple html.
If you want label a field, you can use the placeholder tag which works in most modern browsers:
If you want to pre-populate a field, with text, you can use the value tag which will put the text, but NOT remove it when the focus is put on the field: