Sources are the documents or records that verify family information, events, dates, and relationships.
All records are subject to interpretation and can contain errors during the recording of the information. Also, a person may have supplied the incorrect information on purpose. Such as a date of birth on a military or marriage record, so they would appear to be old enough to enlist in the military or get married.
Sources are rated for the facts that can be extracted. Each source provides different information and should be rated as such. Not every source is easy enough to figure out how to rate, sometimes it is personal opinion. Another difficulty that arises is from not knowing who supplied the original information, what the intent was, and how accurate is the information? This is one reason why you need to do exhaustive research, to make sure you have the right document for the right person. One source cannot answer questions of assertion.
If you are unsure of how to rate your sources and need more clarification, consult books and internet documents for primary and secondary sources, and genealogical proof standards. Two sources to consult:
Board for Certification of Genealogists. The BCG Genealogical Standards Manual. Orem, Utah: Ancestry Publishing, 2000
Rose, Christine. Genealogical Proof Standard: Building a Solid Case. 3rd Edition Revised. San Jose, California: CR Publications, 2009