A Disambiguation Page is one that provides links to other articles in order to prevent confusion. This is done when two or more articles share the same title. For example, Atlas is a disambiguation page which provides links to Atlas (BattleMech) and Atlas (system).
A separate disambiguation page is often set up when there is one dominant usage of a word, but multiple secondary usages. This disambiguation page would then be linked to from the primary article.
Sometimes there is only one secondary usage, so a link is created from the primary article to the secondary article without needing a disambiguation page. An example of this is Procyon. Since the dominant usage is for the planet, that article is there without any parenthetical notations. The main article then links to Procyon (ProtoMech), as it is much less frequently used.