Distillation will remove almost any impurities from water - heavy metals, poisons, bacteria and viruses plus of course all solids. This is actually the best water purification available and it is also very cheap to run. With a compact and easy-to-use distiller like SmartStill you can easily manufacture enough absolutely pure drinking water for a normal household.
What distillation cannot remove is any substance which boils at a lower temperature than water - these substances will vaporize before the water and come out first. Typical such substances are oils, alcohol, petroleum etc.
If you know however that your water contains an amount of any such substance - let's say you have a mix of alcohol and water - then you can simply separate out the first lot that comes out of your SmartStill distiller. That first lot will be the alcohol (or any other substances boiling off first). This is actually how alcohol distilling is done with a SmartStill (where legal of course). The next lot that comes out will be (theoretically) the pure water.
For simple alcohol distillation, SmartStill works very well and it will produce good, pure alcohol around 65-75% strength. Notice though that you should only practice this in countries where legal.
For professional distillation of alcohol, petroleum, oils etc - this "distilling in temperature steps" is taken further. By adding what is called a column, you will get real fractional distillation. The column is a tall tube put on top of the distiller and inside this tube you have obstacles for the vapor, preferably cold obstacles (like cold water running through in pipes). The vapor will condense on the obstacles and drip downwards again - but it will then meet more vapor from below, heating it up again and there will be a second boiling, and third, and fourth etc as it climbes up in the column. These repeated condensations purifies the liquid even further and home distillers using this method are often called reflux stills.
The remaining impurities is left behind with a small amount of water in the boiler and should be thrown away. You will need to clean up your SmartStill now and then from these impurities which otherwise will stick in the boiler and possibly also in the condenser (cooler). Use one SmartClean sachet for removing all this once every 2-4 distillations.