During the trial period, you can use all functionalities of the plugin for 30 days for free and with no obligation.
If you decide that you want to continue using the plugin beyond the trial period, you can already buy a subscription while the trial is still on-going. This will ensure a smooth transition. Of course, you will only start paying the subscription fee once the trial period is over.
Note: You may only use this free trial offer once for each plugin. Also, once you have bought a plugin as a subscription, a trial is no longer possible.