It means when you create harm you feel bad feels that stay in your mind forever and poisons you. And you will never be free from this until you forgive/ get forgiveness. Every evening try to remember anyone who made bad things to you and you unconsciously want to kill him or anyone you harmed without a reason and ask for forgiveness. Even after first such day you'll feel real FREEDOM inside. That's the thing that is good for you, not for people who made you bad things first of all.
buddhism philosophy is good. I've been going to a local buddhist temple for meditation guidance. however, I don't believe in karma, dharma, nirvana, etc. I guess you could call me a secular buddhist