"That is (to say)" in the sense of "that means" and "which means", or "in other words", "namely", or sometimes "in this case", depending on the context. The abbreviation may be followed by a comma or not, depending on the style of the writer (or the grammatical sense of what follows). The comma is more apt to be dropped before a simple expression with no punctuation of its own, and is more likely to be retained for multiple items. I.e. is often confused with e.g. (exempli gratia, 'for example'). Some writing styles give such abbreviations without punctuation, as ie and eg.