唱歌 is a separable verb (离合词) meaning 'to sing a song'. The two syllables 唱 (chàng, to sing) and 歌 (gē, song) can be split apart when you add a measure word, duration, or other complement between them — for example, 唱了一首歌 (sang a song). Unlike a regular verb, you can't put a direct object after 唱歌; instead, you insert the object between the two parts. In everyday conversation, you can also use it as a unit (一起唱歌).