An agent is a separate individual who performs much of this filtering process. You most certainly don't want to send a manuscript directly to a publishing house. They won't read it.
They consider pieces only if they come recommended by an agent. Agents read manuscripts, or ideas for manuscripts (known as queries and proposals), and decide whether a project has promise.
If it does, the agent signs a contract with the author, promising to use best efforts to get the thing sold to a book publishers , in exchange for around 15% of the deal. Editors at publishing houses would much rather deal only with agents who have a good track record for presenting quality ideas, so agents can be very choosy about who they sign.
Landing an agent, therefore, is the whole idea of the game. Once you have one of those on your side, she will work incredibly hard to get your idea sold.
Agents, in turn, don't particularly like reading 300-page manuscripts either. In fact, they don't like reading much more than 1 page. So the first step to getting an agent to even pay attention to you is to send them a query letter. A query letter is essentially a short summary of your idea, who you are, and why you are qualified to write this project. Or you could try the self publishing route which is also valid.