What they are
Primary sources allow researchers to get as close as possible to the topic being researched. They are documents, artifacts, firsthand testimony, images, and more, that provide direct evidence of the topic involved. If you're researching a historical event, a primary source is created by someone who was there, and directly experienced the event. A primary source could also be something like a song, movie, poem, or speech. In these cases, it's a primary source because it's an original work created by someone, and not an imitation, replication, or interpretation of an original work.
As an example, a diary from an immigrant from Ireland to the United States documenting their travel experiences from Ireland to New York City would be considered a primary source for research on Irish immigration to New York City. A book written by a scholar that analyzes the various writings of Irish immigrants and interprets the experience of those immigrants is a secondary source for the same research topic.