A colloquial contraction of 'ということは' meaning 'that is to say' or 'so that means', used to draw a conclusion from previous information.
彼が来ないってことは、予定が変わったのかな？
If he's not coming, does that mean the plan changed?
ってことは君が全部払うの？
So that means you're going to pay for everything?
今日雨なら中止だよね。ってことは、明日に延期？
If it's raining today, it's cancelled, right? So that means it's postponed until tomorrow?