A more thorough explanation:

Mere right is an abstract right in property that does not include possession or even the right of possession. It is also known as jus merum, merum jus, or meer dreit.

Imagine that John inherited a piece of land from his father, but he never took possession of it. Later, his cousin Tom moved onto the land and has been living there for several years. Even though John has the true ultimate property of the land, he only has a mere right because he does not have possession or the right of possession. Tom has obtained the absolute right of possession through his occupation of the land.

This example illustrates how a person can have the true ultimate property of a piece of land but still only have a mere right if they do not have possession or the right of possession. In this situation, the person with possession has the absolute right of possession, and the person with the mere right must prove their ownership to recover the land.