Both terms come from Mircea Eliade’s work, and are primarily outlined in his book Patterns In Comparative Religion, though he does not specifically define either term.
"Reductionism" is attributing "the sacred" to something other than "something religious. " (Eliade, Patterns, xiii) As examples, describing the sacred as a function of economics, sociology, politics, evolution, or any other social science is to reduce it to a "dependent variable" that changes based on other factors. (Pals 161)
"Anti-reductionism" rejects social-scientific attempts to explain the totality of religious experience casually. It accepts that all religions, regardless of state (modern, primitive, archaic, etc.) are complex and deep. Religion and the sacred are constants: other variables (such as economics, politics, social evolution, etc.) must be seen to depend on it, not the other way around. In short, religion is a cause, not an effect.
When we consider ritual from an anti-reductionist standpoint, we see ritual actions as related to the sacred rather than related to social actions. As an example, a priest who stands on a dias high above his congregation may be seen as representing a divinity, not as asserting himself over his congregants in a political manner. Likewise, the sacrament of Confession in the Catholic Church can be seen as a reflection of divine forgiveness, as a sacred act, rather than as a social outlet for the reduction of guilt (or the sharing of guilt among the community).