The Chrono Trigger is a Time Egg, and is used to save Crono from the Ocean Palace Incident.
Lucca gave a Time Egg to Kid as a keepsake. Lynx demands that Kid give it to him during the final confrontation. Interestingly, Lynx states that using a Time Egg with the Frozen Flame will allow control over all history.
Note, Demiforce translated it as the "Chrono Trigger", but the official translation (including the Japanese source) merely calls it a Time Egg.
A legendary item that is a complete enigma. It is believed to break the bonds of time.
A Time Egg is a mysterious and enigmatic device capable of allowing time travel. The only individuals in history capable of creating them are Belthasar, Gaspar, and Lucca (though her prototypes were incomplete). The materials involved in its construction are unknown, as are its mechanics, though it most likely utilizes arcane magic to function. Specifically, it utilizes miniature black holes; according to Lucca's notes, "by rotating a single point of supergravity, space-time continua can be drawn in, thus making it possible to transform that singular point which pulls in everything else into a ring formation. Using this ring as a Gate between dimensions, it is be possible to travel back and forth between various space-time." The Time Egg can be then be described as a self-contained Gate, allowing the user to travel to whichever era he or she pleases. Activating the Time Egg is a different matter. The Chrono Trigger was such a device, and several stipulations surrounded its activation. These were spelled out by Belthasar and Gaspar; specifically, they noted that the Time Egg would have an effect equal to the amount of effort given toward the achievement of its intended purpose; that if it were being used to save a person from death, that the person in question must be important to the space-time continuum; that a clone would be needed to satisfy the second requirement.
The veracity of these stipulations is unknown, as it would suggest that the Time Egg has the ability to differentiate important persons in the scope of history. The second note about one's importance to the continuum may have been simply a warning against causing unnecessary historical change by preventing death. Curiously, Crono's friends also did nothing to activate it except release it into the air and speak their desire. In any event, the extent of the ability to use a Time Egg at will is unknown; when Serge used one to travel to the Darkness Beyond Time, its purpose had been previously spelled out by Belthasar as well. Another mystery involves what happens to the Time Egg after its use. After Crono's friends deployed the Chrono Trigger atop Death Peak, it shattered, though soon had the intended effect of taking them to the Ocean Palace. (In Radical Dreamers, Kid's Chrono Trigger shattered after its use as well.) The fate of Serge's Time Egg is also unknown, as immediately after it is used, Serge fights the Time Devourer and the Unification of Dimensions occurs. While it is possible that Time Eggs are meant for one use only, it would seem as if the labor and magic skill going in to their creation would be a tremendous waste. If they are indeed only able to be used once, it is perhaps a testament to the power of The Entity that it was able to make permanent Gates.
Lucca worked on building Time Eggs before her house was raided by Lynx and burned down; she was able to fashion an incomplete version that she gave to Kid. This incomplete Time Egg was able to function, but the effect was temporary. When Kid passed into a coma after lapsing into the events of the orphanage fire, Masa & Mune were able to charge the incomplete Time Egg with enough power to send Serge back in time. However, as their power drained, Serge and his party members returned to the modern era. The incomplete Time Egg may also have been used in conjunction with the Pendant to guarantee Kid's salvation in perilous times; while Kid remarks that its her pendant that rewinds time when she's in danger, the incomplete Time Egg may come into play as well simply because the pendant has never exhibited any temporally transforming properties. Time Eggs continue to be shrouded in mystery; even the description in the Cross menu acknowledges this, calling it "a complete enigma...believed to break the bonds of time."
Theory and Analysis
From: Key Items (Chrono Cross)