A more thorough explanation:

A selective-draft law is a law that allows the government to force citizens to join the military. This means that if the government decides that there aren't enough soldiers, they can make people join the military whether they want to or not.

For example, during World War I, the United States passed a selective-draft law. This law allowed the government to conscript citizens for military duty. This meant that if you were a young man during that time, you could be drafted and sent to fight in the war, even if you didn't want to.

The constitutionality of the first selective-draft law was challenged and upheld in the Selective-Draft-Law Cases. This means that the Supreme Court decided that the law was constitutional and could be enforced.