To be helpless is to have no ability to take care of or protect yourself from harm. A newborn baby is helpless, and needs parents to care for her.
If you're helpless, you're dependent on other people to assist or care for you. Breaking both your legs will leave you helpless, and even being in an unfamiliar city can make you feel helpless as you wander around looking for your hotel late at night. You can also use helpless to mean "irresistible" or "uncontrollable," especially when someone bursts out laughing: "They broke into helpless laughter during his speech."