The concept of 'primary' and 'secondary' sources is key to studying and writing history. A 'source' is anything which provides information, from a manuscript where words tell you things to clothes which have survived centuries and provide details on fashion and chemistry. As you can imagine, you can't write history without sources as you would be making this up (which is good in historical fiction, but rather problematic when it comes to serious history) Sources are usually divided into two, primary and secondary. These definitions would be different for the sciences and the below apply to the humanities. It's worth learning them, they are vital if you're taking exams.
A 'Primary Source' is a document which was written or an object which was created, in the time period in which you are working. A 'first hand' item. A diary can be a primary source if the author experienced the events they recall, while a charter can be a primary source of the act it was created for. Photographs, while beset with problems, can be primary sources. The key thing is they offer a direct insight into what happened because they were created at the time and are fresh and closely related.
Primary sources can include paintings, manuscripts, chancellery rolls, coins, letters and more.
A ' Secondary Sounrce' can be defined two ways: it is anything about a historical event which was created using primary sources, and/or which was one or more stages removed from the time period and the event. A 'second hand' item. For instance, school textbooks tell you about a time period, but they are all secondary sources as they were written later, usually by people who weren't there, and discuss the primary sources they used when being created. Secondary sources frequently quote or reproduce primary sources, such as a book using a photograph. The key point is that the people who made these sources are relying on other testimony rather than their own.
Secondary sources can include history books, articles, websites like this one (other websites might be a primary source to 'contemporary history'.)
Not everything 'old' is a primary historical source: plenty of medieval or ancient works are secondary sources based on now lost primary sources, despite being of great age.
Sometimes you will see a third class: the tertiary source. These are items like dictionaries and encyclopaedias: history is written using both primary and secondary sources and shrunk down to the basic points. We've written for encyclopedias, and tertiary is not a criticism.
One of the primary tools of the historian is the ability to study a range of sources and assess which is reliable, which suffers from bias, or most commonly which suffers from the least bias and can be best used to reconstruct the past. Most history written for school qualifications uses secondary sources because they are effective teaching tools, with primary sources introduced and, at a higher level, as the dominant source. However, you can't generalize primary and secondary sources as reliable and unreliable.
There is every chance a primary source can suffer from bias, even photographs, which are not safe and must be studied just as much. Equally, a secondary source can be produced by a skilled author and provide the best of our knowledge. It's important to know what you need to use. As a general rule the more advanced your level of study the more you'll be reading primary sources and making conclusions and deductions based on your insight and empathy, rather than using secondary works. But if you want to learn about a period quickly and efficiently, selecting a good secondary source is actually best.