"That is (to say)" in the sense of "that means" and "which means", or "in other words", or sometimes "in this case", depending on the context; may be followed by a comma, or not, depending on style (American English and British English respectively). It is often misinterpreted as "in example." In this situation, e.g. should be used instead. There should be a period after both letters, since it is an abbreviation of two words.