A more thorough explanation:

Definition: A son-in-law is the husband of one's daughter.

Examples:

• My son-in-law is a doctor.
• She introduced her son-in-law to the family at Thanksgiving.

The examples illustrate the definition by showing that a son-in-law is a man who is married to someone's daughter. The first example tells us that the son-in-law is a doctor, which is additional information about him, but does not change the fact that he is married to the speaker's daughter. The second example shows that the son-in-law is being introduced to the family, which is a common occurrence when someone gets married.