Where the Random Explorer generates a collection of completely random worlds, the Series Explorer generates a series of more or less similar worlds.
To activate the Series Explorer manually, click the Series radio button. When World Builder starts up, it fills the Series Explorer edit boxes with figures that should generate a collection of world maps that are different enough to be interesting, but similar enough to be obviously related.
To start generating worlds, click the 'Start' button on the toolbar below the main Explorer display window.
If this is the first time you have generated a set of worlds after starting up World Builder there will probably be a few seconds delay. The main Explorer display will then start to fill up with thumbnail world maps. You can either wait until the Explorer finishes, or you can click the 'Stop' button to stop exploring.
Note that the world maps are all relatively similar (but should be different enough to be distinct).
The Step value determines how different the worlds are. The smaller the Step value, the more similar the worlds will be. A step size of 25000 (the default value) usually produces a good spread.
The From and To figures are the 'seed' values for the range of worlds. These values can be very large (up to about 900,000,000,000). When you are using large values, keep checking the message at the bottom of the Series section, to make sure that you are not going to generate ludicrously large numbers of worlds!