In history you have no doubt come across, and had to use primary sources. Primary sources offer a wealth of information, but they do have their limitations. There are a number of reasons why you cannot take the words of a primary source on face value:
1. Reason: Why is the historian writing what they are writing? What style of writing is it? Fiction, non-fiction? Prose, poetry?
2. Social: Where in society did the writer belong? Was he a low lying peasant or was he a rich beurocrat? This can link to how he perceives the society he is writing about, be it a top-down or bottum-up view.
3. Political: A lot of history is about politics. Depending of what side of the political spectrum, be it right wing or left wing, could influence their writing, and thus what you are reading. Be cautious of this.
4. Economic: This links with the aforementioned point of society. Where they are in society affects how they would interpret it.
5. Geography: Where someone was born, or where they have lived, or where they where when they wrote the piece you are reading can have an influence on their interpretation of events. If you are reading a British extract of wartime conditions in World War II, you will only recieve a British perspective, thus making it useless if you wish to include an interpretation from another nations soldiers etc.
6. Bias: Every writer has an opinion and it will always show in their writing. Even if a writer claims to be neutral, it very rarely is.
Source analysis is a big part of history, and becomes a much greater deal if you choose to take your history education further.