If you're recording speech, it's no surprise to find the best position for the mic is in front of the mouth! That's why, even in the 21st century with all our technology, singers still hold the mic to their mouths - nothing else will do.
But the camera doesn't want to see a big microphone in the shot so we compromise. We take the mic 'out of shot', but then it's too far away and will pick up unwanted background noises. So we use gun mics with special directional or 'zoom' abilities. These mics 'hear' more in one direction than others and help get the sound you want to go with the picture and reject what you don't want. Getting the mic in the best position is VERY important for getting the best sound.
what's the boom pole for ?
Let's look at some typical camera framings.
wide shot
when you can see all of someone, including their feet, this is usually a 'wide shot'. Where would be the closest place for the microphone to get to the mouth, yet stay out of shot? Yup - over the top. You're going to get a very sore arm if you don't use a pole.
If there is bright sunlight or direct lighting, the mic may cast a shadow - experiment to find a place where you can get close enough to the mouth but not make a shadow or get in shot
positioning the boom pole
A medium shot usually means you can see someone from their head to their waist. Look again for the shortest distance from the edge of the frame to the mouth.
medium shot
With a close up your options improve. The bottom of the frame might even be closer to the mouth BUT people do have a habit of waving their arms around. And if they're walking about as well then you might find you're in their way. So the best place to try the mic to start with is always over the top.
close up