The alcohol is protonated by the HX acid to form its conjugate acid. This step is necessary to make the OH group a better leaving group.
The halide ion (X-) attacks the protonated alcohol, displacing the water molecule and forming the alkyl halide. This step is the rate-determining step of the reaction.
Primary alcohols react with HX via an SN2 mechanism. This means that the reaction occurs at the carbon atom to which the OH group is attached, and the stereochemistry of the product is inverted.
Secondary alcohols can react with HX via either an SN2 or an SN1 mechanism. The SN2 mechanism is favored under acidic conditions, while the SN1 mechanism is favored under more neutral conditions. The SN1 mechanism can lead to racemization of the product.