Slide film will have a certain latitude, or range of stops (zones) it can comfortably "cover". Some people call it a "scene brightness range" or SBR. Often, the important parts of what you want to shoot will be within that range. If so, then it's great. But, sometimes the contrast is higher or lower. Then you can use a different film if you have it with you. That's why there is Astia and Velvia - one tones down the contrast and the other really bumps it up. As far as the basic idea, it is the reverse of print film. For a negative you expose for the shadows (zone 3). With slides, you expose for the highlights (zone 7). A lot of times you then have to decide what you want to keep, and what you will allow to block up or blow out.