Growing up, I heard this idea many times but it often failed to resonate. Though, as I've spent more time on this road as a psychologist, I've begun to appreciate it's message.
I've learned that words shape our stories. Like when a client describes their relationship as "not having any problems." I've learned that language constructs our reality. Like when we tell ourselves that things will never get better, no matter how hard we try.
So take a moment and think about the words you say - to yourself, to others, and perhaps, about others too.
Take a moment to think about what might be different if instead of saying "bad," you said "not good." Try it out, play with it, and notice what changes.