The Multi-Select Lookup Field enables you to establish what we call a many-to-many relationship between two modules in CRM.
For instance, consider the example an insurance company. A single policy may be held by many policy holders. Similarly, a single policy holder may hold many policies.
If you have two modules - Insurance Policies and PolicyHolders - there is a need to associate multiple records from each module to each other. This is not possible with a regular look-up field, as it allows only a one-to-many relationship.
In this case, you can use a Multi-select lookup field from the New Fields tray in the Layout Editor. This will let you associate multiple records from two modules to each other, thus establishing a many-to-many relationship in CRM.