You are sperm. Sperm is what fertilizes the egg, creating you. When people say this, they're saying
your mother should have instead given head, and swallowed the sperm which was cummed in her mouth, instead of having
regular sex, and having you.
You are that sperm, so if she would have swallowed the sperm, she would have swallowed you. It's basically like saying your dad should have had sex with a condom...if someone tells you this, they're calling you a mistake, and looking down on you as a useless lifeform. It's actually a very clever line if used as an insult.