Slide transitions can be set up to govern how you move from one slide to another.
You can set your presentation to be timed so that it moves automatically to another slide after a certain length of time. This effect is used for informative presentations which are just left running for people to view as they walk past.
However, it is difficult to get automatic timing just right, some people read fast, some much more slowly. They can lead to frustration if some are kept waiting for the next slide for too long whilst others find they haven't read everything before the slide switches.
You can set the slide transition to different effects such as 'fade', 'blinds', 'checkerbox'. This can be set up on every slide so that the transition is consistent throughout the presentation, or it can be set up on particular slides only.
As with custom animation, the rule 'less is more' applies and it is wise to stick to just one effect throughout the presenation.