Leading is the amount of space between lines in a paragraph of text. I guess this is sometimes called line spacing. Too much or too little space between your lines can make the text difficult to read. Sometimes when you make your text really big, the space between lines increases too much. It becomes too big. In slideware the leading is usually set by default to about 20 percent larger then the font size. So if you use very big fonts in your slides you may have to adjust the leading (or line spacing) to make related text closer to each other so they do in fact appear related.
Below is a before and after showing a slide with the default leading (in PowerPoint) and a slide with adjusted leading. The amount of adjustment you must make depends on the size of the font used and a visual inspection of the result of the adjustment. Remember too little space is also bad.
Image Credit: Spatial Mongrel