Removing and replacing water is called a water change.
A "cycled" aquarium is an aquarium that has built up lots of the good bacteria mentioned above. They live on surfaces like the glass, decorations, and substrate, as well as the filter cartridge- which is why you should only ever clean your filter cartridge in a bucket of tank water, never with tap water. If your tank is cycled, you can keep fish. If your tank is not cycled, you cannot keep fish because the ammonia is burning their gills. They will most likely die, and even if they don't die, they are in considerable pain.
Also, unless you're using well water, you need to add water conditioner to remove chlorine.
Please read up on the basics of fishkeeping.