Meaning: This sentence describes someone who is good at running and swimming.
Choice of Words: Fast means moving quickly. Runner is a person who runs. Good means satisfactory or desirable. Swimmer is a person who swims.
Alternative Expressions
She is a fast runner.
Meaning: This means that she can run very quickly.
Choice of Words: The word "fast" means moving quickly. "Runner" is a person who runs. When you put these two words together, it means that the person is good at running quickly.
She is a good swimmer.
Meaning: This means that she can swim very well.
Choice of Words: The word "good" means being able to do something well. "Swimmer" is a person who swims. When you put these two words together, it means that the person is good at swimming.