what does it mean to port and polish and the engine?

I am actually new to mini bikes and new to this forum in general. Recently, I bought a used x19, and I had a problem with the idle because it would shortly die afterwards. However I found the idle screw and I turned it to the right a bit and now the idle seems to be fine. Back to topic though I wanted to know what it means to port and polish a motor will this increase air flow or give more power? So far I have only gotten my bike up to 40mph, but I also don't really have room to go faster than that.

in a quick rough overview, you grind down and sand down/polish the inside of the head and intake, especially around the valves and it will cut down the air resistance for the air/fuel in, and then the exhaust gases out. faster flow means less sluggish motor, meaning you go faster. how much this will help performance varies by the motor, the design, the quality of workmanship, and how good of a job you do. go too far and there could be gaps, meaning less compression, meaning losing power.
also keep in mind if you get 3% gain of a 7.5hp motor, chances are the only noticable change will be what you imagine in your head.

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I think its a waste of money. Your intake and exhaust ports, is like a garden hose. What happens when you just let a water hose run? Water comes out a slow-normal pace. Now when you put your fingers over the nozzle of your garden hose, you make a smaller path for the water to travel through, thus making the water travel faster. The same theory applies to the ports. Open them too much and your going to lose power because the intake is traveling slowly into the head. If you make the ports smaller by a little there is be proved that a few hp's are gained. But go too small on the intake port and your just restricting it.