A more thorough explanation:

Definition: A writ used in medieval England to protect an undertenant from being harassed by a higher-ranking lord for rent that was actually due from the middle lord. This writ was called "de medio" in Law Latin, which means "of mesne."

Example: Imagine that you are a tenant who rents a piece of land from a lord. However, that lord is not the highest-ranking lord in the area. Instead, there is another lord who is above your landlord, and that lord is demanding rent from your landlord that he has not paid. If the higher-ranking lord tries to collect that rent from you, even though you have already paid your landlord, you can use a writ of mesne to protect yourself from being harassed.

Explanation: The writ of mesne was used to protect tenants from being caught in the middle of disputes between their landlords and higher-ranking lords. By using this writ, tenants could avoid being harassed or forced to pay rent twice. The example illustrates how the writ of mesne would be used in a situation where a tenant is being unfairly targeted by a higher-ranking lord who is trying to collect rent that is not actually owed by the tenant.