It's the beginning of a phrase, and the meaning will be completed by the rest of the sentence. For example:
* Oui je suis d'accord: Yes, I agree.
* Oui je veux: Yes, I want (to).
* Oui je comprends: Yes, I understand.
It's important to note that "oui" is already a complete answer meaning "yes," so "oui je" is used to emphasize the "I" part of the response.