The d’Hondt system is a system of highest averages for working out seat allocation. It ends with a proportional result, though can be distorted through small constituency sizes. In short, you give a seat to the highest party, then divide their total votes by the seats they have plus one. Then, at that stage whichever the party with the highest number is gets a seat. So in the chart below, you can see one example.
For a less abstract concept, watch the video below: