The rule states that when an unsymmetrical reagent like HX like HCl,HBr is added to an unsymmetrical alkene then negative parts of the reagent i.e halogen X(Cl-,Br-) goes to the carbon atom carrying smaller number of hydrogen atom.

Reaction :
CH3-CH2-CH2-CH=CH2 + HCl → CH3-CH2-CH2-CH(Br)-CH3, when 1-pentene reacts with hydrochloric acid to give 2-Bromo pentane.

 
When an unsymmetrical reagent HCl is added to an unsymmetrical alkene i.e. 1-pentene then negative parts of the reagent Cl- goes to the carbon atom carrying small number OH hydrogen atom.