Use this feature if you want to allow someone else to start a meeting or webinar you schedule if you're unable to. This is a good idea in the event that you are out of the office or late to your meeting or webinar. You can also designate a departmental account as an alternative host. When you designate an alternative host, that user or departmental account will receive an email notifying them that they've been added as an alternative host, with a link to start the meeting or webinar.
You can also give another person or departmental account scheduling privilege over your Zoom account, which will make them an alternative host for all of your meetings and webinars and allow them to schedule meetings for you. They can also schedule webinars for you as long as neither of you are students. Read more about Scheduling Privilege.
Note: If you are in the Healtcare Component (HCC - BAA/BAA+), then your alternative host(s) must also be in the Healtcare Component (HCC - BAA/BAA+). Likewise, if you are not in the HCC (i.e. you are non-BAA), then your alternative host(s) cannot be in the HCC either.
See the Alternative Host article in the Zoom Help Center for step by step instructions with screenshots on designating an alternative host based on the device you're using to schedule your meeting.