Ref-Def is shorthand for Reflection Deflection, a memory-wiping spell that works in conjunction with special S.M.I.L.E. mirrors. These were introduced in a small MLP novel that explains Lyra’s inclusion into S.M.I.L.E. affairs.

@Background Pony #97E7  
Yup. S.M.I.L.E. is sort of the MLP world’s MiB, and Lyra and Bon Bon have to keep a lot of Equestria’s wildest secrets under wraps. In the Gameloft game specifically, an incident with Dr. Hooves one Hearth’s Warming causes a time stream disruption, and Lyra and Bon Bon have had to work together to keep the fabric of Equestria’s time stream together without getting caught. This has included characters from the ancient past appearing in the present, key events in the present happening over again, ponies from the future appearing in the present, etc.

Here come the Men in Black  
They won’t let you remember

Wait, “memory-wiping”? Bon Bon can do that?

Wordbubbles in words

tf is happening to Bon’s left eye