In MySQL INDEXES or KEYS without constraints are oftenly called "Normal Indexes", "Ordinary Indexes", or "Non-Unique Indexes". Normal Indexes are the most basic indexes having no restraints such as uniqueness.
Sometime fields need to index only for faster search results against them and also they can not be designated as primary or unique fields because they may contain duplicate values. Because this is a common place occurrence, it only makes sense that those fields should be indexed without constraints. MySQL allows to create Normal Indexes. These are simple to create and much like creating indexes of other types.
Normal Indexes or Keys Related FAQs