Sources are remains from the past that historians study to learn about that past.
So, a source = information.
For example: journalists use different sources to write their articles: documents, eye witness stories, photographs, video footage etc. These are all sources (of information).
There are different types of sources. For example:
Written Sources: every object from the past that has textual information: books, inscriptions, maps, coins.
Non-written Sources: every object from the past that has no textual information: bones, weapons, buildings, clothes.