Think of primary sources as raw data, and secondary sources as the analysis of that data.
Primary sources are contemporary, first-hand accounts/individual representations and works of events, topics, or historical periods. These are usually experienced and created concurrently. A primary source is unfiltered information, meaning there is no analysis or outside review.
Examples of primary sources: letters, diaries, personal papers, memoirs/autobiographies, newspaper articles, speeches, dossiers, reports, artifacts (tools, weapons, ornaments from the time), works or art, audio/visual recordings, etc.
Secondary sources are built off of the primary sources. These summarize, evaluate, and/or analyze the content within a primary source. Secondary sources allow for consideration and formation of new perspectives and interpretations of the impact of an event/period/etc.
Examples of secondary sources: textbooks, monographs (single-topic book), research articles, reviews, theses, dissertations, biographies, data analysis, commentaries, etc.