Don't Attach When Attacking
"Don't Attach When Attacking" is the negative way of saying Attack from a distance.
In the case where there are no other stones around, if White wants to attack the black stone, attaching to it (playing tsuke) will prompt Black to extend to a, or to play hane at b.
If Black extends to a, he has strengthened his stone and White finds himself stuck on a wall which is stronger than he is.
If Black hanes at b, White finds himself with a lack of liberties whereas Black can make shape if White responds.
For better ways of attacking, see How to attack.
In both these cases, the black stones will be stronger after the attack than before, whereas White will be weakened.