What do you mean by dehydrohalogenation reaction ?

Explanation

When Ialoalkanes with B-hydrogen atom are boiled with alcoholic solution of potassium hydroxide, they undergo elimination of hydrogen halide (HX) resulting in the formation of alkenes. For example,

These reactions are called B-elimination reactions because the hydrogen atom present of B-position of haloalkanes i.e. at the carbon atom next to that which carries the halogen is removed.