Artefacts are content items that do not contribute to the meaning of the content. Very typical examples of artefacts are parts of a page that are a result of pagination - page number, running header and footer - but also purely decorative objects, like a page background, or any purely decorative elements in general. Usually it does not offer any value to people with disabilities to be presented with such objects if all they care about is the actual content of the document.
MadeToTag offers two keyboard shortcuts that make it easier to deal with artefacts:
On the screenshot we can now easily see that the chapter number '1' - but nothing else on this page - has been tagged as an artefact.
Note: The current version of MadeToTag does not hide master page items when hiding artefacts. A future version will hide master page objects in the same way as artefacts.
Let's assume for this exercise that the big "1" to the left of the title of the article is just meant as a decoration, and does not belong to the real content. In order to tag it as an artefact, select it, call menu item Object -> Object Export Options... and under Tagged PDF set Apply Tag to Artefact.
Do the same for any other items that are artefacts.
Important note: Objects on master pages are always artefacts - and Indesign CS5.5 always exports them accordingly. So you do not have to tag objects on master page as artefacts.