The "To" field includes the records you selected before clicking on the "Email" button or you could choose to send your email to everyone in your account of the specified tab (i.e. Volunteers, Opportunities, Organizations or Clients). This is also where you can add other email addresses or insert a "Mail Merge Field" that contains an email address (which is denoted by the word "email" in its title).
Unless you specify certain records, the default setting is to send to all of the records.
Only "Mail Merge Fields" that have the word "email" (or "e-mail") in it can be inserted into this field. Also, only this kind of Mail Merge Field can be inserted into the other email address fields such as "From," "Cc," and "Bcc."