A rebuttable presumption is an assumption of fact accepted by the court until disproved. All presumptions can be characterized as rebuttable. It is an assumption that is made in the law that will stand as a fact unless someone comes forward to contest it and prove otherwise.
For example, in adoption law, it is most commonly used to "presume" that if a woman is married when she gives birth to a child, that her husband is its father. This "presumption" will stand as a legal fact unless it is contested and proven to be wrong.