他们在聊天。

They're chatting.

Source: Tatoeba

Sentence Composition

1 他们 tāmen they; them
2 在 zài
  1. to exist; to be alive
  2. (of sb or sth) to be (located) at
  3. (used before a verb to indicate an action in progress)
3 聊天 liáotiān to chat; to gossip