As the name says the Current Target Window represents the current target to be used on every actions with Target Current.
By default the Current Target will shows the distance between you and the target, the health status of the target and the main context menu actions.
The Current Target will be automatically removed when it's beyond ~20 tiles of distance (that's because targeting anything above that distance, even if you can see it with a high zoom level, will cause the game to crash). But when the the target returns in range, it will be re-engaged automatically.
Like many windows this one can also be locked and is affected by scale and alpha mode.
Every target will also have this X button to close the current target.
Every target also have the auto-target button. When used, you can toggle the auto-target function (same as using the Auto Target Current action). When active all cursor target will be automatically used on the current target as soon as it's in rage.
NOTE: You can use the option Use "Auto Target Current" to Re-Engage Current Target to trigger your cursor target when you're current target gets out of range to re-engage it without wasting time and targeting it manually.