Popular question this week...
Maddy,
Cycling the tank is the process that grows the healthy bacteria in your aquarium (in the filter sponge and media, the substrate, on the decorations), that turns the ammonia (from fish poo, pee and excess food) to nitrItes and further breaks it down to nitrAtes. NitrAtes however, are also harmful to fish, so when you do water changes you are diluting the nitrAte concentration to safe levels for the fish, <40 ppm and <20 ppm is ideal.
The price of the kit can range, (in my city at least), between $20.00 - $40.00 USD. It can be used many many times and is more accurate than test strips; an all around good investment.