It is a margin to the oscillation stop and the most important item in the oscillation circuit.
This margin is indicated by ratio based on resistance of crystal, and it shows how amplification capability oscillation circuit has.
The oscillation circuit can theoretically operate if oscillation margin is 1 or more. However, if oscillation margin is close to 1, it is possible that operation failure will occur on module by excessive long oscillation start up time and so on. Such problems will be able to be solved by larger oscillation margin.