publishing is like a high-security facility.
And because of the way we tell stories (i.e., we skip the boring bits), it's easy to hear "breaking in" as synonymous with having "arrived."
What does it really mean to break in?
It might mean many things. The one thing it doesn't mean is that you've made it.
In terms of the market that we call publishing, it means that you're now a player.
At the most fundamental level, it simply means that someone is willing to make a non-trivial investment in your work.
- Getting an agent, for example, means that he or she is investing their time and energy because they believe they can land a contract for your book and get paid.
- Getting a contract means that the publisher is investing real money, both by financing your advance and through the cost they will bear, in your book.
- Getting readers who will buy your book means they are investing some money and, more importantly, time in your story.
Breaking in only means that you're invest-able. There are no guarantees, for any of the parties involved, that the investment will pay off.
As with many things in this delightfully perplexing industry, "making it" is really a euphemism for "starting a whole new game."