This term refers to the practise of including extra keywords into a web page to boost its rankings in the search engines. This includes hiding keywords on the page by making the text very small, colouring the text with the same colour as the background, hiding keywords in comment tags, overfilling alt tags with long strings of keywords, etc. Keyword stuffing is a way to fool the search engines and, as such, its use should be strongly discouraged as such practises can lead to serious search penalties if discovered.