回 huí means: to return.
In the combination 回来 huí lái (= come back; lit.: return, come), 来 lái (= come) indicates a direction towards the speaker.
In the combination 回去 huí qù (= go back; lit.: return, go), 去qù (= go) indicates a direction away from the speaker.
In these combinations, 来 lái and 去 qù are therefore called complements of direction.
| 他 | 回 | 来 | 。 |
| tā | huí | lái | _ |
| he | return | come | _ |
| he, him, other | toward the speaker: come back, return | 。 | |
| He comes back. | |||
| 他 | 回 | 去 | 。 |
| tā | huí | qù | _ |
| he | return | go | _ |
| he, him, other | go back | 。 | |
| He goes back. | |||