Fresh water (also 'freshwater' or 'fresh-water') is water that does not have a lot of salt in it.
When people say 'fresh water', they usually mean water from the lakes, rivers, snow, and ice, which is not salty. It also can mean water that people can drink. The oceans and seas are made up of salt water, which people cannot drink.
However, people cannot drink all fresh water, because it might be contaminated. It might have harmful bacteria in it, or poisonous because of chemicals it contains. Water from a tap has been tested and treated so that it is safe to drink. This applies to most countries, but in some place the water needs to be boiled first, and drunk after it has cooled down.
Fresh water that people can drink safely is called 'potable water'. The way in which many developing countries turn salt water into drinkable water is that they use desalination.