Power creep can be summed as this: You start out weak, and get stronger. New content comes, and you get only stronger, but enemies don't pose much more of a challenge than they did before. More content comes, you get even stronger and more power, but because enemies don't scale like you do, they are weaker. The best way to represent this in Destiny was Forsaken and the entire year up to Shadowkeep. We had absurd power creep. Well of Radiance, sniper buffs, grenade launcher buffs, bosses were getting shredded and we were practically immortal by the time Opulence rolled around. Even the heroic Menagerie basically depended on cheesing guardians to achieve any form of difficulty, in the form of strong enemies and mechanics that required high risks. And even then, people found ways to solo it by the end of the season. Basically, power creep causes the game to be in a state where the players are always too strong and new content is not challenging enough or has to be made absurdly difficult in order to stop players from burning though content.