Secondary sources are works that are not based on direct observation of or evidence directly associated with the subject. They rely on other sources of information. They may also be works that comment on another work, such as a review, criticism, or commentary.
In refining a research topic, we often begin with secondary sources. They help to identify gaps or conflicts in the existing scholarly literature that might prove to be promising topics.
Primary (historical) sources are documents, images, objects, or other records that provide firsthand accounts or direct evidence about a historical topic or event. These records were created at the time of an event or were later recalled by an eyewitness; or they were created by someone with a direct connection to the topic.
Primary sources in the sciences can be different from primary sources in the humanities and social sciences. In the sciences, the term "primary sources" often applies to original research or the first (i.e., primary) article to report on new research or data. However, many archives do not retain journal articles as they are readily available from libraries and other digital sources. Review this guide from Michigan State University for more on the topic.
Primary sources emphasize the lack of voices and interpretations between the thing or events being studied and reports of those things or events. There is a belief that firsthand accounts are more accurate, but that is not always true.
Primary sources can be digital. Many archives digitize some of their holdings – both for preservation and for access. You don't have to physically handle sources to use them, though that is part of the fun.
For more, check out this guide to primary sources from Princeton University.