A tactical reload is the process of replacing your current (although not necessarily empty) firearm magazine with a fresh magazine, in particular the one that is hardest to reach. Typically, the expended mag is stored in a pocket or dump pouch.
It’s the opposite of a speed reload. In a speed reload situation, you replace (but not also store) the fully empty magazine by retrieving a fresh one from the easiest-to-reach position.
The benefit of these two types of reload processes is that in each you keep your optimal sight alignment and picture, while retaining your dominant hand’s position on the trigger.