A secondary source interprets and analyzes primary sources; it helps you to understand relevant primary sources. Secondary sources are one step removed from the event. 
In the field of law, law reviews are a secondary source; the one below, for example, analyzes the Kelo v. City of New London Supreme Court decision.
Other examples of legal secondary sources: textbooks, case citators, legal dictionaries & encyclopedias, journal articles, legal digests.
Image source: Heinonline.
A primary source is a document or other sort of evidence written or created during an event or time of study (or after the event, if done by an eyewitness). Primary sources offer an inside view of a particular event. 
In the field of law, primary sources are the actual text of the law, that is, legislation, opinions, case law.
Other types of primary sources: diaries, speeches, official records, scholarly journal articles reporting new research or findings.
Image source: Kelo v. City of New London. 2005. US Supreme Court website.