An ordinal scale is a measurement scale that assigns values to objects based on their ranking with respect to one another. For example, a doctor might use a scale of 0-10 to indicate degree of improvement in some condition, from 0 (no improvement) to 10 (disappearance of the condition). While you know that a 4 is better than a 2, there is no implication that a 4 is twice as good as a 2. Nor is the improvement from 2 to 4 necessarily the same "amount" of improvement as the improvement from 6 to 8. All we know is that there are 11 categories, with 1 being better than 0, 2 being better than 1, etc.
See also: nominal scale, interval scale, ratio scale