Copyright infringement is using someone else's work without getting that person's permission.
The author of any original work, including books, essays, webpages, songs, pictures, and videos, automatically gets the copyright to that work, even if he/she does not label with with the copyright symbol and their name.
You need to worry about copyright when you are preparing a presentation using images or music.
Many images or pieces of music are copyrighted and you can't just take something that you find on the internet and just use it in your presentation.
There are sites which provide copyright free images or music.