Females have two X chromosomes in each cell except the sex cells.
Only one is expressed and the other in activated. It has a transcriptionally inactive structure called heterochromatin.
This inactivated X is also called the Barr Body. It can be seen microscopically and is used to identify if a person competing in sports is female.
X-chromosome inactivation is a random process.
The inactivation process is best understood by looking at the coat color of female cats. The black and orange colors are on separate X chromosomes.