I went to 'Sign Up' to use this site and ask questions. Put in a user name and password. The next option was 'Sign In'. It came back with a blank 'Sign In'. Keeps doing the same thing. I think I can post this as a guest. Can I post questions indefinitely as a guest? Where are the instructions for signing up? Should be simple and would be if it worked. What does '300 reputation' mean? Can someone help with tags for this?
It was probably a bug that you couldn't sign up. I sometimes have this on my mobile phone where it prevents me from logging in. But normally it works when I come back a couple hours later.
You can post indefinitely as a guest as long as your questions / answers are not deemed to be spam or always very low quality. Your profile tracks the questions / answers / comments / ... you post and the system will send you a message when someone replies, for example by answering a question of yours. You will lose this feature if the cookie you are using as a guest is lost because you won't be able to login again. See What does it mean when someone's an Unregistered User? for more information. You can ask the staff via the contact link at the bottom to merge your accounts once you created a full account though. See How can one link / merge / combine / associate two accounts / users? (Anonymous / unregistered / cookie, or Google / Facebook / registered)
You get reputation for asking and answering questions when people upvote the stuff you post. There are a few other ways, such as suggesting an edit while you have less than 2,000 reputation, accepting an answer, having an answer of yours accepted, ... Reputation allows you to interact with the site in more ways. 5 reputation is normally the minimum to post on the meta site for example and 50 reputation allows you to comment. It's a measure of how much you have contributed to the site and how much the system trusts you that you can use the tools given to you responsibly. See How does “Reputation” work? for more information.