The YC application form has fields that cover this. You should talk about your grand ambition in the "What is your company going to make?" answer, then describe your progress so far in the "How far along are you?" answer.
Which Y Combinator companies focus on finance?
When did South Park characters stop calling things "gay" in the vernacular derogatory sense "not very good"?
How accurate is the GitHub status site?
Very accurate. Every time I've noticed a problem with GitHub the status site has either already published it, or publishes within a minute or so of me first noticing.