The left side on your browse screen has a rolodex type of icon. This indicates a change from one "record" in the database to another. If you had a client name field for example, you could click on this icon and visually see one name, which is one record, and then the next and so on. If you had a global field in this same scenario, the contents of this field would not change from record to record. So global is a constant value from one record to another. When you use a global field will depend on whether or not the contents are should be the same from record to record.
I hope this helps.
Marty