Simulation Coverage

Hi, I have a few questions about the simulation coverage percentage.

What exactly does it stand for? I already read the definition in the Altera knowledge database, and did not understand it.

Will my design work if this percentage is low? I heard that the lower the percentage, the more likely my design will fail, but I'm not sure if this is true.

What can I do to increase this percentage?

Sorry if the questions are to basic, but I am new to this.

Thanks a lot.