Pulsation Modes of Cepheids There are various modes of Cepheids like


single modes: 
   fundamental (F), 
   first overtone (1O), and likewise.


And then we have 


double modes: F/1O, 1O/2O etc.
triple modes: 1O/2O/3O etc.


What are these modes actually? What do they mean and which frequencies do they correspond to?

Stellar pulsations are caused by expansions and contractions in the outer layers as a star seeks to maintain equilibrium. What kind of equilibrium is being referred to here?
Some classical Cepheids pulsate simultaneously in two or even three modes.
Their lightcurves can be explained as a overposition of fundamental plus overtone modes.



The terminology you used is also found in this paper (where you have also other references):
"the discovery of many double-mode Cepheids (DMCs) pulsating in both the fundamental and first-overtone modes (F/10) and the first and second-overtone modes (10/20)"

Other reference: Classical cepheids