If a person has negative behavior, behavior modification may help him/her adopt positive behaviors to replace the negative behaviors.
For instance, my younger daughter used to whine to get what she wanted from me. By ignoring her whining and reminding her to ask nicely, she soon changed her negative behavior.
When my brother picked up a nasty swear word when he was a pre-schooler, my mother taught him to see phenylbarbitrol when he was angry. She helped him replace an undesired behavior with acceptable behavior.
In simple terms, behavior modification is replacing socially unapproved/ineffective behavior with behavior that is more approved or effective by not reinforcing (ignoring) negative behavior and reinforcing more approved alternative behaviors.
Only punishing undesired behavior has a couple of drawbacks. First, it only indicates what NOT to do, rather than showing them what to do. Second, the attention they get when being punished might be reinforcing.
"Catch them being good" is a slogan that suggests you reinforce desired behaviors. Too often approved behavior is ignored, because it is "expected." It is much better to ignore the undesired behavior and reinforce the desired behavior.
In everyday life, praise and "thank you" are effective and easy secondary reinforcers. Whenever I see a child put some trash into a trash can (especially at a picnic), I make some comment like "Good job!" to reinforce that behavior. Praised behavior is more likely to be repeated.
Modeling also helps. At the same picnic, if I see someone throw trash on the ground, I pick it up and put it in the trash, often suggesting to the person that it would be better to put it in the trash can.
This article might explain it in more detail:
I hope this helps a little more.