To get clean sound you have to get a good uni-directional mic (one that records what's in front of it) close enough to the actor delivering the dialogue, so that you get good, clean levels... good levels are -12db
The sound has to be loud enough to be clean, but not so loud that it goes to the maximum and clips (a clicking sound made when too much volume goes into the recording device) So, it's a bit like setting exposure for light, but with sound: too much is bad, too little is bad... you need, just right.
Generally, the mic needs to be less than three feet from the actor and pointed directly at their mouth. If you have two actors in the same shot, you have to give the boom operator time to twist the pole, so the mic is pointing at the right actor when the line is delivered.
In order to get the mic this close, in most cases the boom operator needs to hold the boom over his head, with his arms extended. His job is to get the mic as close as possible, without getting the mic in shot. This is very hard work... and this is why the pole needs to be both very strong and very light.
There are other problems... if the pole is knocked, the vibration travels down it and into the microphone, creating a rattling sound. Or, if the cable knocks against the pole, the same thing happens... you also get bad noise if the boom op twists the cable against the pole.
To solve these problems, the mic is usually attached to the pole by a "shock mount" which is just some rubber cord criss-crossed to form a cradle. This like the shock absorbers in a car, in that if the pole gets knocked, the mic doesn't get disturbed.
On top of these problems is the problem of wind (not the too many chilli dog kind, but the moving air kind)... mics are very sensitive to air movement, because that's how they record sound. They have a membrane that vibrates when the air pressure changes ... unfortunately the mic can't tell the difference between sound and wind. So, the mic has to be protected inside a fur material covered cage... because, strangely the fur absorbs the wind, but not the sound... in the trade this is sometimes called a "fluffy" or a "hamster" or ... well, the other names probably aren't suitable for an open forum... but I think you can work it out, if you remember that it's something furry that sound men are fond of! (and before any of the Yanks tells me they've never heard a crew use these names, remember that I'm British and we use different, and often funnier, terms on set... and that I work with the most foul mouthed crew in the known universe)
It's possible to build DIY version of all the pro-kit (except the mics)... but, it is worth remembering that the pros will pay well over $400-$2,000 for the windshield and fluffy alone
(here are some example + they sell boom poles)
Hope this helped... I tried to balance language use with technical detail... tricky at the best of times