To query is to ask or inquire about something. http://dictionary.reference.com/browse/query
Most publishers or literary agents ask that you send a query letter to them before submitting a synopsis, sample chapters, outline, or proposal.
The query letter should give them an idea about what you've conceived, experienced or want to explore or explored in your writing. The information you present in a query letter may appeal to one agent but may not to another. So this may be subjective, but that's alright because there are many agents and publishers who may like your idea or your work.
Sometimes an agent may like your query letter and request to see more of what you're writing. However, as a writer, don't build up your writing dreams after this request. The agent may not like what you submit and you'll have to submit it to another agent. In other cases, the agent may like what you send and this will be the start of major shift in your writing career.
Good luck with the query letter - I still have to get to that stage of getting my query letter accepted. For now though, I'll continue to "fix-up" my work.
See Nelson's link (http://www.nelsonagency.com/faq.html) for samples of great query letters with explanation to guide you.
Here are some links that discuss writing a synopsis.
Here are some links about book proposals:
Sample Book Proposal
How to Write a Book Proposal
Preparing Your Book Proposal