Primary sources are documents like letters, diaries, sermons, music, interviews and news articles written by people living through an event. They are often useful to help you identify the issues a person or people thought were important. They help you see opposing viewpoints surrounding an issue.
Secondary sources are typically scholarly articles and books written to analyze a topic or issue.
Tertiary sources are resources like encyclopedias, biographies, fact books and textbooks that summarize major topics. They are useful to help you find basic background information on a topic for further research. For example, you might read an encyclopedia article on a topic to learn who were important people involved with an event. Then you can look for primary sources by those people and secondary sources about those people.