da does mean 'good', but it never means 'very'.
The course notes explain that on its own 'iawn', as taught so far, means 'OK', 'fine', and so on. However when used following an adjective such as da it acts as an intensifier 'very':
- da iawn - 'very good', 'very well', or sometimes 'well done' - it all depends on the context.
- cyflym; cyflym iawn - fast; very fast
- hen; hen iawn - old; very old