All original content, or reuploads of pre-published content that you own copyright for, are immediately licensed as under the Creative Commons CC BY-SA 4.0 license upon upload. This is for example the same license family used by Wikipedia.
Any non-freely licensed content that you upload and do not own copyright for might be considered fair use. Fair use are murky waters. Wikipedia takes a very strict approach of very limited fair use: en.wikipedia.org/wiki/Wikipedia:Non-free_content, but we are more relaxed to it, and only take gray cases down upon copyright owner request.
Some examples of what should generally be OK:
  • quote up to a paragraph from a copyrighted book, clearly attributing it
  • explain what you've learned from a book or course in your own words.
    You also have to take some care to not copy the exact structure of the original, as that itself could be subject to copyright.
    One good approach is to just use several sources. If multiple sources use the same structure, then it is more arguable that this structure is not a novel copyrighted thing.
  • use a copyrighted photograph when there is no free alternative to illustrate what you are talking about
If the copyright owner complains, we might have to take something down, but as long as you are not just uploading a bunch of obviously copyrighted content, it's not the end of the world, we'll just find another freer way to explain things without them.