that is to say

我们需要早点出发，也就是说，早上6点前。

We need to leave early, ___that ___is ___to ___say, by 6 AM.

图书馆六点闭馆，也就是说，我们只剩二十分钟了。

The library closes at six; ___that ___is ___to ___say, we have only twenty minutes left.

这条路线禁止汽车通行，也就是说，我们必须步行继续前进。

The route is closed to cars; ___that ___is ___to ___say, we must continue on foot.