He suffers from anterograde amnesia, meaning he can't form new memories, and relies on tattoos and Polaroid pictures to track his progress. His quest is driven by the desire to find his wife's killer and avenge her death.
However, the film cleverly plays with the unreliable nature of Leonard's memories and perception. As the story unfolds, we learn that he may be manipulating his own investigation, possibly even causing Sammy's death himself. The true nature of the crime and Leonard's role in it remain ambiguous, adding to the film's unsettling and thought-provoking nature.