EatAust says...
It tends to apply to perennial veggies in the same family and has to do with maximising soil fertility and breaking disease and pest cycles.

1. Diseases and pests affect a plant but can be carried in the soil from year to year. By planting something from a plant gamily that isn't affected by that disease or pest, the pests and diseases cannot thrive.

2. It allows plants with different fertility needs to use the same batch of soil in a cycle, thereby increasing your yields in that one patch without you having to artificially change the soil's nature.

The Australian fruit and veggie book I got from the Digger's club has excellent examples of what plant families to follow which in a seasonal cycle, but I've had a brainsnap and simply can't remember a blessed thing! Nitrogren-fixing legumes being followed by nitrogen-hungry plants are a classic example, however.

Others will have more practical examples :)