The reaction mechanism that is shown here is an SN1 reaction
What is SN1 reaction?
A nucleophile is used in place of a leaving group in the nucleophilic substitution reaction known as the SN1 reaction. It is distinguished by a first step that is slow and unimolecular, in which the leaving group dissociates to form a carbocation. In the second step, this unstable intermediate quickly reacts with a nucleophile.
Good leaving groups and stable carbocations—which are frequently created by tertiary or secondary alkyl halides—are preferred by the reaction. If the carbocation is chiral, it may result in racemization.