This expression is often translated as es decir or o sea. However, it is also used to explain or correct something previously said, and it has a slightly different meaning.
I felt very tired this morning. That is, until I had my first coffee— Me sentía muy cansado esta mañana. Eso sí, hasta que tomé mi primer café
I plan to buy a yacht when I retire. That is, if I am still alive— Pienso comprarme un yate cuando me jubile. Eso es si aún estoy vivo
Bolt looked like he was going to win the race. Until he slipped and fell, that is— Parecía que Bolt iba a ganar la carrera. Hasta que se resbaló y se cayó, claro.