A social host is anyone who provides alcohol to others. Essentially that's anyone who's ever had a party or just had a few friends over. Social hosts can be subject to liability for serving alcohol to guests.
But it's technically not the act of serving that can cause a problem, and in California the law doesn't apply to just any guest. Social host laws are designed to solve a specific issue.
What lawmakers don't want is for underage people to get access to alcohol and then cause damage or injuries. That means people who serve under-21 guests can get in trouble.