What does it mean to say B is representing the dominant gene

Dominant genes are always represented by a capital letter which in this case is the letter B whereas recessive genes will be represented by a lower case letter like b.

You only need one copy of a dominant gene or allele to inherit it. A dominant allele is always expressed even if only one copy is present.

But you need two copies of genes or alleles to inherit the recessive gene. There also can't be a dominant allele of the gene if you are to inherit this.

Allele - Different forms of a gene

smooth muscle is composed of involuntary muscles found within the walls of organs and structures such as the esophagus, stomach, intestines, and blood vessels. those muscles push materials like food or blood through organs. unlike skeletal muscle, smooth muscle can never be under your control.