You've probably seen scripts that have notes down the right side of the page to describe what the transitions between the scenes will look like. These include "CUT TO:" or "DISSOLVE TO:" or the ever-popular "SMASH CUT TO:" and "MATCH CUT TO:". Those last two create a specific visual effect, so many authors like to use them to make their imagery more vivid. I know one outstanding screenwriting teacher / screenwriter who insists that his students write transitions in their scripts, but he’s the only one that I know of and I have no idea why he teaches this. In my opinion, you never need to include any transition words in a spec script.
The first problem with transitions is that they constitute yet another attempt to direct the filming of the story, which is the director’s job. Your job is only to tell a dramatic story with compelling characters, and those rarely include the words “dissolve to.” The other problem is that you want to make your script as brief and active as possible, so cutting out unnecessary elements like transitions can only help you accomplish that goal. Your shooting script can include cool visual elements like transitions, but for your spec script, just stick to telling the story.