Setting up a guest account allows outsiders to view your calendar and propose events or meetings.
To use it, make sure the radio button next to "Enable Guest Account" is set to "Yes". The guest's login username is set to match your username and you can't change it. You can assign guests a password or leave it blank. Most people leave it blank since guests can't cause much damage. They can enter events with a Status of "Proposed" and that's it.
Email them the URL below the "Guest Account Link" text for logging in. Best is to copy and paste it.
Once they post a proposed event, the calendar owner (matching username) automatically gets an email notification. You can click on the link in the message to change the event status to "Confirmed" or "Cancelled", the guest who proposed the message automatically getting a message either way.
The guest account is a great way to deal with the common need to schedule meetings when the other person(s) are unable to commit to a date/time, usually because they don't have their calendar with them. You tell the other person(s) to login as your guest when they do have their calendar, and schedule a meeting on an open date & time, perhaps while conferring among themselves. This avoids the volleys of voicemail while people propose meeting dates & times that the others can't make.