Here's a story that I've heard more than once. A photographer becomes interested in film photography again. He or she digs around in their closet to find that old 35mm camera that they know is in there somewhere.
Once it's retrieved, they test the shutter: yes! But alas, the light meter isn't working. Either it's no longer functional, or it requires batteries that are difficult to find. Enter the Sunny 16 Rule for exposure.
Simply put, in bright sunlight, set the aperture to f/16 and the shutter speed to the ISO of the film. So if you're shooting T-Max 100, then it's f/16 at 1/125th of a second. This rule can be adjusted to various lighting conditions. The bottom line is, if you don't have a functional light meter, you can still use your beloved film camera.
The Phoblographer has published a terrific article that explains the ins and outs of this technique: The Ultimate Guide to the Sunny 16 Rule: Part 1. Even if your light meter is working, there are benefits to understanding the Sunny 16 Rule. Batteries can drain unexpectedly or your light meter can be fooled. This is the ultimate backup plan for exposure.