Oxymercuration is a process by which water is added to an alkene through treatment of the alkene with mercury(II) acetate [Hg(O2CCH3)2, usually abbreviated Hg(OAc)2] in an aqueous tetrahydrofuran (THF) solvent. Rarely this reaction is referred to as Oxymercuration/reduction. This reaction looks just like Markovnikov. addition of water across a double bond, however there is no carbocation intermediate, so there is no rearrangement. You will learn that NaBH4 is a common reducing agent.
Addition of water by oxymercuration