What is dehydrohalogenation reaction?

Hint:Dehydrogenation or dehydration describes the removal of hydrogen atoms whereas, dehalogenation describes the removal of halogens. Therefore, as the name suggests, dehydrohalogenation suggests the removal of hydrogen and a halogen group.

Complete step-by-step answer:Let us describe the dehydrohalogenation in detail;
Dehydrohalogenation reaction-
It is in general, the synthesis reaction of alkene (or alkyne) from alkyl halides; by the elimination of HX (H represents hydrogen and X represents halogen) from adjacent atoms.
The alkyl halides or haloalkanes are usually the bromo- or iodo- or less commonly chloro- substituted alkanes.
It is normally carried out in methanol with sodium methoxide, in ethanol with sodium ethoxide or in tert-butyl alcohol with potassium tert-butoxide; along with the presence of heat.
One of the reactions can be represented as;
n-butyl chloride gets converted to but-1-ene in the presence of alcoholic KOH.
$C{{H}_{3}}C{{H}_{2}}C{{H}_{2}}C{{H}_{2}}Cl\xrightarrow[Heat]{KOH\left( alc. \right)}C{{H}_{3}}C{{H}_{2}}CH=C{{H}_{2}}$

Note: Do note that this reaction would be more precise if we call it a beta-dehydrohalogenation reaction as the hydrogen is being removed from the beta carbon i.e. the adjacent carbon to the carbon, halogen is attached.
Also, if more than one alkene is produced during the reaction then, more substituted alkene is the major product of the reaction.