Usually, to access these functions, depress the exposure release button halfway. Aim the focus point at an object the same distance as the subject of your exposure. Now hold the shutter release button until you want to take the picture. This will keep the camera focussed at that distance. Why would you want to do this? Suppose you are taking a picture of a moving object (such as a child at a soccer game). You can't focus on your subject because your subject isn't where you want them to be. So you pre-focus on the spot where you will take the picture. You don't have to wait for the camera to focus or wonder if it focused, because it's already done.
The same action controls the exposure lock. In fact exposure-lock and pre-focus are in effect at the same time. Examples of situations where exposure-lock might be useful are: backlighting, contrasty light, and snowy scenes. The light meter in your P&S (and all other cameras) wants to make everything look medium grey. It will make snow grey and black rocks grey. Sometimes you don't want things to be grey. These are instances when you would use the exposure-lock feature. In order to use the exposure-lock feature aim the focus point at a medium grey object or scene. Green leaves look grey to the camera's meter. Asphalt pavement is close to medium grey as is some tree bark. Try to see things in black and white and average out the values in your head. Once you've found a suitable subject, at the correct distance (because the pre-focus will be triggered at the same time), depress the shutter release halfway, point the camera at your actual subject, and release the shutter.