谈天 is a separable verb (离合词) meaning 'to chat' or 'to have a casual conversation'. The verb 谈 and the object 天 can be separated by aspect markers, duration, or other elements — for example, 谈了一会儿天 ('chatted for a while'). It is an informal, colloquial word, similar to 聊天. You cannot place a direct object after 谈天; instead, use 和/跟 + person + 谈天 ('chat with someone').