Weapons are an intrinsic part of Aikido training. They are used to extend your understanding of Aikido fundamentals.
The three traditional Aikido weapons are the tanto, the bokken, and the jo.
The tanto is often referred to as a 'knife', but in fact it is simply the smallest of the three traditional weapon designs based on the sword. The tanto is technically a short sword, The blade varied from 6 to 12 inches. In Aikido training the shorter lengths are most often used. Tanto training involves learning various techniques involved in taking, or retaining the tanto.
Traditional tanto training is NOT intended to replicate modern knife fighting techniques, it is a method of improving your core Aikido skills.
The bokken is most often associated with Aikido weapons training. Bokken literally translates into 'wood sword'. Technically a tanto is a bokken, and you can get wakizashi length bokken, but people generally mean the long katana-length sword when using the term bokken.
Bokken training involves retention and taking like tanto training. It also involves kata and suburi training; as well as paired exercises. Like other weapons training, bokken training is intended to improve your holistic Aikido and not make you an expert in sword fighting.
The jo is somewhat unique in that it is the only weapon used in modern Aikido that was originally designed as a wooden weapon rather than a wood representation of a weapon.
Traditionally the jo is a medium length stick weapon. The length allows it to be used as a staff, a sword, or a baton. It is still in use as a weapon by some law enforcement personnel in Japan.
Jo training mirrors bokken and tanto training. Like them it is used to learn and practice fundamental Aikido concepts. Uniquely for Aikido weapons, it can also be studied as a stand-alone martial arts discipline.