Both paid and free password managers generally require that you use a strong and unique master password to access your account. Since everything is secured using a single password, someone can gain access to your password manager. This is why you must keep your master password to yourself and use something difficult to guess.
Most password managers can help you come up with a string of words, numbers, and symbols for you, which you can use as the master password.
In addition, some free password managers may allow you to use a biometric login, such as a fingerprint scan or facial recognition when logging in. This will give your password vault extra security.