A "cycle" is the nitrogen cycle.
Ammonia is created from die off -> nitrite feeds on the ammonia -> nitrates feeds on the nitrite.
Only when you test 0 Ammonia and 0 nitrite is your cycle "complete" keep in mind that a new tank can have many small cycles in the beginning, so even if you are testing 0 one day, the next morning you might have new values.
I would really suggest reading some of the material here
http://canreef.com/vbulletin/showthread.php?t=22067 before you get to far ahead of yourself.
Check out this link for a bit of an explanation of the nitrogen cycle
http://faq.thekrib.com/begin-cycling.html