What is a "pick-me girl," and why is the concept so harmful to women?

Conversation about the "pick-me girl" has exploded online in recent years. Our culture has fueled it from both ends, by encouraging pick-me behavior and by empowering everyone who can type in the hashtag #pickme on TikTok to criticize the behavior when they see it—which is perhaps why it has nearly six billion views and counting on the social media platform.
Upon learning the definition of a pick-me girl, and almost certainly upon witnessing one, it's easy to make a swift and unforgiving verdict that collapses the complexities of feminism. But our knee-jerk negative response to the so-called phenomenon exposes an even darker underbelly of gender inequality, one which reflects our own criticism back at us. 
What is a "pick-me girl," and why is the concept so harmful to women?