This step allows to put the photos like slides on a light table.

You just have to select the pictures one by one (one click by photo maintaining down the CTRL key) or by list (a click on the first picture, then, maintaining down the Shitf key, a click on the last one). Then, you can choose the power of magnification (with the pop-up menu - on the right - or the magnifying glass icon).

This step isn't really useful for astrophoto, barring you need to eliminate the photo suffering from significant defects. For that, put the mouse over the picture and click the yellow cross appearing (see on the right).
We can go straight to the following step.