It's stuffing non-related with content just for the sake of trying to draw traffic. So that tricks the visitor and makes them land on a web page that they weren't even looking for. Also, t can be over-optimizing your site/webpage with a ton of keywords even though they may be relevant. You should make sure that keywords have a nice balance & flow with your content. If it reads like you're just trying to hard to stuff in keywords for the sake of ranking, then you probably are.