The term dhamma has multiple meanings in Buddhism. Dhamma is defined in the accesstoinsight.org glossary page as:
dhamma [Skt. dharma]:(1) Event; a phenomenon in and of itself; (2) mental quality; (3) doctrine, teaching; (4) nibbāna. Also, principles of behavior that human beings ought to follow so as to fit in with the right natural order of things; qualities of mind they should develop so as to realize the inherent quality of the mind in and of itself. By extension, "Dhamma" (usu. capitalized) is used also to denote any doctrine that teaches such things. Thus the Dhamma of the Buddha denotes both his teachings and the direct experience of nibbāna, the quality at which those teachings are aimed.
If you want to know more about Dhamma, the doctrine and teachings of the Buddha, see this page.
If you want to know more about dhamma (mental quality or mental object), the fourth foundation of mindfulness, see this page, this page and the Satipatthana Sutta.