That is to say... / In other words... / I mean...
Used to clarify, rephrase, or explain what was just said. Very common in both spoken and written Spanish.
The deadline is Friday, that is to say, in three days.
It's an informal proposal, in other words, it's not binding.
—What do you mean? —In other words, I disagree.
He's a polyglot, that is, he speaks several languages.
We'll arrive on Friday, that is, the day after tomorrow.