"Run back, and tell the fortune-teller this message: her time is almost up..."
The termination code was a code Rama-Kandra gave to the Merovingian in exchange for his daughter's safe passage into the Matrix. The code contained certain information which allowed the Merovingian to terminate and destroy the Oracle's outer shell.
The Merovingian, according to the Oracle, was like all men with power, which meant his lust for absolute power was boundless. Thus, he ever desired to obtain the Oracle's prophetic powers to see the future. However, he was denied the privilege, and was told that the "gift" had to given, not taken. Consequently, he grew secretly jealous of the Oracle, and sought a way to remove her from the Matrix, especially after she repeatedly infuriated him by taking away such individuals and powers that were under his ownership, such as the Keymaker. Given the termination code, he then destroyed her form, temporarily sealing her off from the Matrix, much to the dismay of the redpills who relied on her for advice and council.
Neo glimpses Rama-Kandra making the deal with the Merovingian when he accompanies Morpheus and Trinity to negotiate the release of the Keymaker in the film, Matrix Reloaded.