Passcodes for Zoom meetings and webinars provide an added layer of security, as attendees will be required to have them before joining the meeting. As an alternative, passcodes can also be encrypted and embedded in the meeting URL, so attendees can join without needing to know it, but the URL for existing meetings will change when using that option.
Refer to the Zoom Help Center article on Meeting and webinar passcodes for instructions setting them up for your meetings.