A more thorough explanation:

The lessor of the plaintiff is the true party in interest who brings an action for ejectment. In common law, an ejectment action was only for the recovery of the unexpired term of the lease. However, conventions of pleadings required the true plaintiff to grant a fictitious lease to an equally fictitious plaintiff in whose name the action would be prosecuted.

John owns a property and leases it to Mary for five years. After two years, Mary stops paying rent and refuses to leave the property. John, as the true party in interest, brings an action for ejectment against Mary as the fictitious plaintiff. In this case, John is the lessor of the plaintiff.

This example illustrates how the lessor of the plaintiff is the true owner of the property who brings an action for ejectment against the tenant who refuses to leave the property.