Tertiary sources are based on secondary sources. Often, they sum up or collect basic information from secondary sources and compile it in one place. Tertiary sources are generally written for people who are not experts in a field. The information they provide tends to be general, and written in clear, easy to understand terms. Examples of tertiary sources include general dictionaries, encyclopedias, as well as commercial products for a general audience. Examples of tertiary sources on Lincoln and the Civil War might include an article in Encyclopedia Britainica on Lincoln's presidency, or Ken Burns' documentary The Civil War.
Videos like these are tertiary because they try to explain complex events in standard terms that everyone can understand. They are less about scholarly communication, than they are about general education. Here is an example of a tertiary source you might use for an exploration of the Battle of Antietam.