Define substitution reaction? Why do arenes undergo substitution reactions even though they contain double bonds?

Hint: A chemical reaction in which an atom or group of atoms in a molecule is replaced or substituted by another atom or group of atoms is known as a substitution reaction. Halogenation is a great example of a substitution reaction.

Complete answer:
The substitution reaction is defined as a reaction in which one chemical substance's functional group is replaced by another group, or a reaction in which one atom or molecule of a compound is replaced by another atom or molecule.
Arenes, like alkenes, have double bonds, but they do not undergo electrophilic addition because this would lead them to lose their ring aromaticity. A carbocation is formed in electrophilic aromatic substitution reactions, while a carbon anion is formed in nucleophilic aromatic substitution processes.
In the field of organic chemistry, substitution reactions are extremely important. There are two types of reactions.
-Nucleophilic reaction
-Electrophilic reactions
A substitution reaction is also known as a single displacement reaction, single replacement reaction or single substitution reaction. The type of atom bonded to the initial molecule differs significantly between these two types of reactions. The atom is stated to be an electron-rich species in nucleophilic reactions and an electron-deficient species in electrophilic reactions.

Note:
Certain conditions must be met in order for the substitution reaction to take place. They're -Low temperatures, such as room temperature, are maintained and the strong base, such as $NaOH$, must be diluted. If the base has a higher concentration, there is a chance that dehydrohalogenation will occur. The solution must be in an aqueous state, like water.