When ever you are searching through the internet you will come across error codes in your browser at some point in time, for the most part you will just skip them to find what ever you are looking for as they are errors on some one else's servers.
The most common ones like 200 is not actually an error, the two hundred means that all is fine, the request has been sent which is what you want, a 301 error code is also not an error this is telling you that a redirect has been placed on this url to a new url as the original url does not exist anymore, you may of changed your url structure and redirected to the new url which is what you should do but most webmasters do not do.
The more serious error codes that need to be fixed if the are caused by your own website are the 400 and the 404 error, the 400 error code could be interpreted in many ways from a wrong url to a corrupted request or corrupted file on your server and finding the cause of the problem is never easy, a 404 error is the file or document is missing and not available, maybe you deleted an article that no longer exists or you changed your menu structure and the url is missing these need to be redirected if possible.
The 500 error code is the most difficult to fix or even find out what the cause could be, this code usually means something has gone wrong on the server, either your server or some one eles server, this could be a programing error or some other problem with the server, the 500 error could also be caused by your browser so clearing the cache and your cookies may fix the problem.
Most of the 500 errors are caused by servers and they maybe permanent or they may fix themselves at some point in time, a 505 error code is a HTTP error, the HTTP Version is Not Supported or the Server does not support the HTTP version specified in the request.
Having a few 404 errors on your website is not a big deal, but when you start to get hundreds then you have a problem as google does not take to kindly to a massive amount of errors coming from one website, and all these errors can cause any google ranking to slip, so you need to fix the problem as soon as you can, a large amount of 404 errors can be caused by seo components that change your url structure so be cautious when using these components.