We use the term "source" to refer to a lot of different things. It can be difficult to tell them apart online (and even on paper!). In the academic context, a source is anything you might read or cite for a paper.
A source could be:
How do you know what kind of sources to choose for your research? Read on...
Use the CRAAP test to evaluate whether a source is worth using.
C = Currency
R = Relevance
A = Authority
A = Accuracy
P = Purpose
In college level writing, an important distinction we make is Scholarly vs. Popular sources. Your ability to tell the difference and choose which is most appropriate is going to be a key skill for your academic career.
Scholarly sources: Also called "peer reviewed" sources, scholarly sources are often required for your assignment. They have been written by professional scholars and scrutinized by other experts in the field. How do you identify a scholarly source? Look for "University Press" in the publisher info, or look up the journal to see if it is a peer reviewed publication. When you are searching Library databases, there is often a filter for "peer reviewed" or "academic journals" you can click.
Popular sources: Almost everything else is a popular source. Most books, news, magazines, and other media can be called a popular source. There's nothing wrong with citing these if you have a great reason! For example, the best sources of information about super-contemporary artists and writers are often in popular sources: magazine articles, videos of talks or interviews, etc. This is because it takes a lot more time for scholarly articles (and books) to be written about them.
Other source types you probably already know overlap with both categories above:
Primary Sources: Original writings, manuscripts, letters, journals, works of art, or other direct results of creative/scholarly efforts.
Secondary Sources: Materials, often found in scholarly journals or books which are interpretations of primary resources, based on first-hand exposure to them.
Tertiary Sources: Materials or writings, found in popular journals or books which are interpretations of secondary resources, at least two steps removed from primary resources.