Record sources are the birth, marriage, death, census, and other records that we find. They document basic facts about that individual, like what day they were born on, when they were married, etc. It is important to "attach" these sources to the individual(s) to document the specifically entered facts. The following statement is often quoted.
Sources document that the individual actually lived. Just because Aunt Mabel told us that the information is true, doesn't mean it is, unless there are sources documenting her facts.
FamilySearch is now doing a lot of the searching for you, even while you are asleep. Just how cool is that? They will show you a record source as a "hint". Just look at it, and determine if it belongs to your ancestor or not and then attach it through their "Source Linker" if it is.
Sources not only have dates and places, but also may have information describing your ancestor's physical characteristics, like having red hair, being tall & slender, and having blue eyes. World War 1 Draft Registration cards are great for this kind of information. Census records will tell you where they lived and often what they did as an occupation and much more.
Below is an example of attaching a record source using the FamilySearch Source Linker. Notice how the names don’t quite match in spelling. Remember that spelling rules didn’t really start being applied until the early 1900s, so phonetic spelling errors were common.