Tug stacking may refer to the process of putting tug modules on top of tug modules to carry a payload or go to other planets. Edit
This may sound silly, but it actually works. Lets put it this way.
Just as you are reaching RED, your fuel runs out. You can do nothing as you pass right by the planet. Next time, you build a rocket with more fuel efficiency. You crash right into the red planet. What do you need? A tug module. You test out your rocket. Just as you reach 88 units from red, you turn backwards quickly and burn fuel to slow yourself down. But wait, you run out of fuel and crash into the red planet! Angrily, you quit the game. You spend time thinking of what to do.. and then.. you come up with the perfect idea! Tug stacking!
An easy way to do it is to use the full Saturn V rocket, but instead of the payload fairing, you place as much tugs as you can fit, and put your payload in.
This strategy can save time, because if you are efficient with your fuel (not going to the top of the velocity bar) it will last from HOM, to PUR, to BLU, to GOL, to the LUN, then RED, to YEL, and possibly any other planets you want to go to. Since you have all those tugs, you can always maneuver yourself and get out of sticky situations.