Does this mean that my site is down for these visitors?
No. It simply means those particular visitors are looking for files which don't exist on the server. Anyone can type something like www.example.com/imdex.html into their browser and generate a 404 error. There are also many bots or agents which scan sites for common files (like 'robots.txt' or 'favicon.ico') which might not be present on the site in question.
If 131 hits is a large number of errors for the site and time period in question (for busy sites, that would be a very small number), look at the actual errors in your server error logs to find out what exactly files are being requested.