There are always dark clouds covering the sun, and tomorrow will be sunny after all. What do you mean?

It means that although we will inevitably encounter difficulties and setbacks in our lives, as long as we keep our faith and keep an optimistic attitude, we will certainly be able to tide over the difficulties and finally usher in a better tomorrow. Cloudy days represent difficulties and setbacks, while sunny days represent a bright future. So the meaning of this sentence is that although there may still be haze and difficulties, as long as we persist in our efforts, we will eventually usher in a better tomorrow.