It's an indicator as to how these fields will behave if you perform a find on them. The dots are really magnifying glasses too:
From FileMaker help on quick finds:
A green icon indicates that the field is searchable. A yellow icon indicates that the field is searchable, but the search might take longer than fields with the green icon. Some examples of fields with yellow icons are related fields, fields that can’t be indexed, or calculation fields that don’t store the calculation results.