Alternative Host is only available on paid licenses. (Pro - Business or Enterprise). Both the booking host and the alternative host have to be licensed
It allows a host to book an alternative host. This is used when booking a meeting on behalf of another host or if the host booking the meeting is able to start the meeting. In this case the alternative host would start the meeting and once the organizer is available, they can jump into the meeting and would receive the host privileges.
The following Zoom Support article, gives further explanations