The 'Trim From at' option allows you to restrict the number of characters that are displayed in the 'From' text field. The 'From' text field holds the email id of the person who sent you the email.
Click the 'Trim From at' drop-down box to view all the options available. You will see numbers up to the limit of 50 characters. Select the numbers of characters you wish to see in the subject field of any email you receive. You also have the option of not trimming the 'From' field.