This feature will not be added automatically to all apps that have in-app subscriptions. Instead it will be up to the developer to decide if they wish to incorporate free trials in their apps. However if the developer decides to go ahead with the feature, users will need to provide a valid payment method and will not be charged until the trial period is over. If the trial is not cancelled before then, users will be charged accordingly.
Given that a lot of publishers have started offering up digital versions of print magazines, offering a free trial could be the equivalent of you standing at a newsstand or bookshop and flipping through a magazine before deciding if you should buy it.