cycling is basically a way to describe establishing the nitrifiyng bacteria in your filter.
see, fish produce ammonia
ammonia is toxic to fish and most living creatures.
there is a kind of bacteria though that break up ammonia into nitrite.
nitrite is also toxic, but luckily, there is also a bacteria to break this up, this time into nitrate, which plants use as fertilisers and it is harmless to fish unless it is present in huge quantities.
these bacteria appear by themselves but as said before, they take about a month to populate your filter.