We derive stringent constraints on the persistent source associated with FRB 121102: size , age year, energy erg, characteristic electron energy the radiating plasma is confined by a cold plasma of mass these properties are inconsistent with typical "magnetar wind nebulae" model predictions. The fact that suggests that the hot plasma was created by the ejection of a mildly relativistic, shell, which propagated into an extended ambient medium or collided with a pre-ejected shell. Independent of the persistent source model, we suggest a physical mechanism for the generation of fast radio bursts (FRBs): the ejection from an underlying compact object, cm, of highly relativistic shells with energy erg and Lorentz factor , into a surrounding e − p plasma with density (consistent with that inferred for the persistent source). For Es similar to observed FRB energies, plasma conditions appropriate for strong synchrotron maser emission at are formed. A significant fraction of the deposited energy is converted to an FRB with duration , accompanied by ∼10 MeV gamma-rays carrying less energy than the FRB. The inferred energy and mass associated with the source suggest some type of a "weak stellar explosion," where a neutron star is formed with relatively low mass and energy ejection. However, the current upper limit on R does not allow one to rule out , or the ejection of a larger mass well before the ejection of the confining shell.