Tap water is water directly from the spigot or faucet. It can either come from a well or utility mains. The idea is similar when you wake up in the middle of the night and need a swallow or three of water, so you stumble to the kitchen to get a cup before returning to bed.
The issue with tap water is that you get whatever is on tap. It is not filtered or chilled. You'll possibly get a nice dose of flouride and maybe lead in some municipalities. It does normally taste better when you filter it and chill it down. Most refrigerators with a built-in water and ice dispenser built into the door have filters and cooling. These have a bonus of making the ice cubes and crushed ice taste neutral instead of like a dentist office.
Iron Noder 2017