Trackers are basically issue types. You can think of them as global categories for the issues you'd like to track.
For instance, in a Planio configuration for software developers, you may find trackers called "Feature," "Bug," and "Task". This means that you'll be able to track features, bugs and tasks separately.
In addition to that, workflows (they define which statuses an issue can have), custom fields and some other properties in Planio are all defined on a per-tracker basis. For example, a feature could be assigned a status of "implemented" while a bug may have a status of "fixed".