Metadata is a way to describe what something is without actually giving the specifics of the something itself.
More people have small screens than big ones.
And if you're planning for the future, consider a 540 pixel width to accommodate TV-screen browsers. And more people have low end graphics cards than high end. Errorlog is a record of all the complaints and groans that the server reports while it's running your site. Like a health chart, you want to read it to check for problems and bad conditions. This will cost some money, but it's well worth the effort.