Broadly speaking, it means a "law", "rule", or "fundamental", and may be used to refer to "rules" of the operation of the mind or universe in a metaphysical system, or to rules of comportment in an ethical system.
Other uses include, in Buddhist philosophy, "constituent factor" in the sense of factors which were first enumerated as constituents of human experience, but then gradually expanded into a classification of constituents of the entire material and mental world. Rejecting the substantial existence of permanent entities which are qualified by possibly changing qualities, Buddhist Abhidharma philosophy came to propound that these "constituent factors" are the only type of entity that truly exists. This notion is of particular importance for the analysis of human experience: Rather than assuming that mental states inhere in a cognizing subject, or a soul-substance, Buddhist philosophers largely propose that mental states alone exist as "constituent factors", and that a subjective aspect is contained in these states themselves.
In Indian logic in general, "dharma" also means "property", used together with "dharmin", "property-bearer". In a Sanskrit sentence like "zabdo 'nityaH" (Sanskrit transliterated according to the Kyoto-Harvard convention), "sound is impermanent", "sound" is the bearer of the property "impermanence". Likewise, in the sentence "iha ghataH", "here, there is a pot", "here" is the bearer of the property "pot-existence" - this just goes to show that the categories property and property-bearer are closer to those of a logical predicate and its subject-term, and not to a grammatical predicate and subject.
See also: Buddhist philosophy