Meaning:

“面” is short for “面见,” which means to “meet,” and “基” is short for “基友,” which means “good friends.” Originally, “基友” refers to “gay,” which is the Cantonese pronunciation of “gay.” However, the term has been used to refer to friends in general, especially among Chinese netizens. Therefore, “面基” often refers to “meeting in person someone you first met online.” Of course, it can also be used jokingly to mean “meeting a good friend of one’s own sex.”

Example:

A: 今天跟我们去吃火锅吧。

Jīntiān gēn wǒmen qù chī huǒguō ba。

Come with us to have hotpot today.

B: 改天吧，今天下班我去面基。

Gǎitiān ba, jīntiān xiàbān wǒ qù miànjī。

Rain check. I am going to mianji (meet a friend) after work today.

A: 为什么叫面基？

Wèishénme jiào miànjī?

Why use this weird term?

B: 因为简单啊，省得说一大堆话。

Yīnwèi jiǎndān a, shěngdé shuō yīdàduī huà。

Because it’s simple. It saves us a long explanation.