In an addition reaction to an alkene, the original double bond between two carbon atoms is converted to a single bond, and each carbon atom that was originally double bonded forms a new single bond...
Br2 + C2H4 → C2H4Br2 OR Br2 + CH2=CH2 → BrCH2CH2Br The name of the reaction is pretty intuitive. Ethene's double bond is broken which leaves room for bromine to be added to form dibromoethane.