Chat gravity refers to the degree of familiarity with another person, which, if you chance to encounter¹ them on the street, will determine if you merely greet them in passing or stop to talk.

聊天重力指你跟另一个人的熟悉程度决定了你们在街头偶遇的时候是只打个招呼就过去，还是停下来聊一会儿。

For example:

We saw each other on the sidewalk and said 'hello' to each other. But she was just a friend of a friend I'd only met once or twice before, and there wasn't enough chat gravity to merit² stopping and chatting.

我们在路边相遇，互相打了个招呼。不过，她只是我一个朋友的朋友，我们也只见过一两次面，所以也就没有足够的聊天重力让我停下来跟她攀谈了。