By tactical combat I usually think the general plan for a shorter term conflict. So for example, if you are about to have a small-scale encounter, such as an encounter in a dungeon, a tactic might be to position ranged combatants and a spell caster at the top of a staircase, while positioning your front line at the bottom to prevent the orcs from reaching them. If your group does this, it is engaging with combat in a tactical manner.
But I think what some people refer to when they mean "tactical combat" in terms of an RPG is:
- There's a battle map with miniatures (as opposed to theater of the mind.)
- Characters have a list of clearly defined "hotbar button abilities" that can be activated to cause various effects. They might be "Whirlwind Attack", "Intimidating Shout" or "Knockdown Strike" (as opposed to looser rules where only basic combat maneuvers are covered, and anything else, like shoving someone away, is up to negotiation with the GM).
Either type of gameplay can be a lot of fun, but to me that's just which rules define how the execution the combat is resolved. The tactical layer is step above that. These rules come into play whether or not either side in the battle have a tactic or not.