From the Ingress Help site:
Resonators must be periodically Recharged to maintain their power and control a Portal. If Resonators lose too much XM, then they will be at risk of easy destruction during enemy attacks. If all the Resonators on a Portal drop in XM health to the Critical level, then Links from the Resonators’ Portal will fail. Recharging a Resonator involves transferring XM from your Player reserve to the Resonator(s) on a Portal.
So, at what level of charge does a resonator go to critical level?