A Sender Email Address or "From" Email address is what your users will see in their Inbox and Calendar as the sender of the calendar invites.
Please pick an email address, you own and have control over (you will need to access this), your users are familiar with, users know and recognize easily.
Most Let's Calendar users pick the same email address from which all of their event communication is being sent to the users.
This is an important step in ensuring your calendar invites reach your users inbox and get added automatically to their calendars.
This also ensure the invite does not end up in SPAM!