When a license isn’t specified, then the work is unlicensed. Meaning, you have not been granted any particular permission to it beyond the terms of the website and what the law provides by default. Generally, that means you can use the work in certain ways and in certain contexts, such as the notion of “fair use” in copyright law, but otherwise your rights to use and distribute it are pretty limited.

There is nothing to think about. All rights reserved except those provided specifically to Y Combinator as outlined by the "User Content Transmitted Through the Site" section of https://www.ycombinator.com/legal/