In some presentation slides, the grid is obvious: a data table is an example. But also without the explicit lines of a table structure, you can recognize a grid in almost any composition you make. A diagram, the positioning of boxes on a slide, even a big picture with a few words of text. Recognize the grid structure, imagine the hidden lines and make sure everything lines up and is spaced out nicely. It will make for a much better slide composition.
If you are ready to dive in to hard core literature on the use of grids in graphics design, I can recommend the 1981 book Grid Systems (affiliate link) by Swiss graphics design master Josef Mueller-Brockmann (some of his poster design are in this Flickr set). The big issue for print designers is to juggle around text columns and images. Presentation slides are a bit different, but still the conceptual approach applies to them as well.
