The iOS comiXology Unlimited 7 Day Free Preview allows you to try out CU for a full week. That Free Preview ends after 7 days and you won't be charged anything at all. The most recent iOS app update starts that Free Preview for you automatically.
If you wanted to continue it, or use CU on any other devices, you can always sign up on our site for the full experience at www.comixology.com.
If you sign up on Android or web, the first 30 days are FREE. After that you'll be charged 5.99/mo.