You can use the phrases "that is" and "I mean" interchangeably to express more clearly what you have just said. You can also use these phrases at the beginning of the sentence as follows:

We had a good time, that is, when he wasn't feeling sick.

You can use the phrases "that is" and "I mean" interchangeably to express more clearly what you have just said. These phrases can also be used before the when-clause as follows:

We had a good time, that is, when he wasn't feeling sick.