By using other modes except for the full manual mode (marked by M), a PowerShot camera will determine the right exposure for you. Basically, that means the camera will set the optimal shutter speed and aperture value that yield the best shot for a particular scenario. Two important things that the camera uses to calculate the exposure are the light condition and the metering mode.
Most of the time, we can just let a camera decide the metering mode. However, under some conditions, it's difficult for the camera to come up with the correct metering. Therefore, we, humans, should know differences between metering modes and be able to choose the most appropriate one for use in a variety of lighting conditions.
Usually Canon cameras come with 3 different metering modes. Let's take a look at each of them.
1. Evaluative metering
Under the evaluative metering, a camera will meter the light information coming from the entire scene. So, there is no weighting or exceptional area. Sometimes, this metering is called an average metering.
In a user manual, Canon suggests that the evaluative metering is suitable for standard shooting conditions, including back lit shots. Normally, in a normal light condition (not too bright or too dark) scenario, this basic metering shall produce a good shot.
2. Center weighted average metering
In this metering, your camera still uses the light information from the entire frame. The only difference from the evaluative method is the center weighted average metering would give greater weight to the center.
This metering is less influenced by small areas that vary greatly in brightness at the edges.
3. Spot metering
This seems to be the most advanced metering mode provided by Canon PowerShot. Spot metering will determine the right exposure by using only meters within the [ ] (Spot AE Point frame) that appears at the center of the screen. That's usually a very small area of the frame.
It's very accurate and is not influenced by other areas in the frame. We use this metering for very high contrast scenes. One example is to photography the moon. Because all area around the moon is so dark, we'll get the overexpose picture if we use other metering systems. Spot metering is our hero for this special scenario. It brings us the right exposure for the moon, but under exposure for the rest of the frame.
In the digital photography world, there are several other metering modes like the Partial metering and Multi-zone metering. Nevertheless, those are rarely found in non-DSLR or compact cameras.
Good luck with your photographing!