It's typically used in response to another parent venting about the challenges of a new stage in her child's development. Exhibit A:
Mom 1 - Jane started to crawl. She gets into everything now. I can't take my eyes off her.
Mom 2 - Just wait until she starts to walk.
They are such competitive words. Oh, you think you have it hard? You think juggling twenty things at once is tricky, do ya? Mom 2 might as well say, I have it harder and I juggle more. Or even, your parenting is pie, I do the real thing. Those are the indisputable implications and they're all really frustrating.
I can't tell you how many times I've heard this sentence from another parent because it happens all the freaking time. You might even know a perpetual offender. I typically respond by affirming the assertion with "I can't imagine!" or "Yeah, that's gonna be hard." I'm a pretty non-confrontational person and I'm very non-confrontational when it comes to parenting. I try to live by the rule that most parents are just trying to survive, just trying to get it done without anyone losing limbs or eyeballs. And so, I bite my tongue and play along.
But, it's not cool. And, maybe more importantly, it's not true. Who's to say that it is more difficult to parent a walker than it is to parent a crawler. There are harder aspects and easier aspects to both stages. A walker has farther to fall to the ground and is, thus, more prone to injury. A walker is also more self-sufficient. It's impossible and short-sighted to characterize any stage as more difficult than another. Each stage of life, and of parenting, has unique challenges.
So, maybe I'll never be able to (or want) to say it in the moment, but I'll say it here - the parenting that I'm doing right now is pretty damn legit and I plan to continue being a rockstar, just wait.