Great question! There are a couple of things to understand about unlimited data plans - the plan will either be unlimited data at full speed or unlimited data with a certain allowance at full speed
Unlimited full speed data - this means you can access what you want, as often as you want to and it will always be at full 4G LTE speed.
Unlimited data, then slowed - these plans allow you to access as much data as you want to (unlimited) but only some of that data will be at full speed. For example, you may have a plan that is Unlimited data with the first 5GB at full speed, after that your data speed may drop to 3G or even 2G speeds
Always check with the carrier before you sign up to an unlimited data plan to make sure it gives you exactly what you need