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.