Check out this example row:
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
| 4/5 | 2 | 8 | 6/9 | 6/9 | 7 | 3 | 4/5/9 | 1 |
Look at cells 1 and 8. Both of those have 4 and 5 as possibilities but cell 8 also has 9. We know a row has to have both a 4 and a 5, and since those cells are the only two places 4 and 5 can go then that eliminates 9 as a possibility in cell 8.
This trick can also be used for a column or square.