It means your host doesnt allow you to have too many people connected to your site at one time accessing data from the database
cheap hosts are notorious for this
from googling the error message which you can also do...
"The reason why are getting this error is because your server is running out of resources. Meaning you have server overload. Simply put, you have too many users in your site for the service you currently have. "
sometimes when google bot is crawling your site there is multiple instances of google bot and you might have a few users too
and a cheap host has you limitied to say "10" connections hit 11 and you will get that error
What version of CubeCart are you using?
could mean your version isnt closing connections
could mean your host doesnt want you having too many connections (probably because they have over sold their DB MYSql servers)
and you should get a better dedicated host