A seer is an individual with the ability to see or predict the future.
The Centauri have an evolutionary advantage over other races, that of de-focused temporal perception; they can - to a limited extent - see the future. For most Centauri, they can predict - down to the year, sometimes down to the day - when they will die. These premonitions are generally unclear. For example, Londo Mollari knew twenty years in advance that he and G'Kar would kill each other, but did not know why nor that he would actually request it off him in order to save their friends from the Drakh. When it came to it, Londo had to at first cajole and then beg his old friend to end his life, since he had believed for twenty years that they would die while fighting each other.
Other Centauri have developed this evolutionary advantage into a considerable ability to see the critical aspects of the futures of other people. Many of these are women, and as such they have earned the title of "prophetess" in Centauri society. Many of the noble houses have an official role of "seer" among their number, usually an older, female family member. This is similar to classic oracles of ancient Greece and Rome, where the art and act of foretelling the future was a legitimate profession, something one devoted his/her entire life to.
There are examples of other individuals with the ability to seemingly predict the future through various means. One example is the Techno-mages, who are able to use advanced technology and statistical science to predict future actions. The technomage Elric was able to foresee the horrors that lay in Mollari's future. The technomage Galen showed the ability - when supplied with the requisite information - to see days or even years into the future, and has even used it to his advantage in order to put crucial players "in the right place at the right time."
Others have shown the ability to predict the future by having extensive knowledge of history and pre-history. The telepath Lyta Alexander, after being modified by the Vorlons, contained a great deal of information locked away in her sub-conscious. With this information, she knew that humanity would be advanced enough to travel to the Vorlon Homeworld in approximately one-million years.