Screen resolution refers to the number of pixels viewed on a user's screen. We know the resolution by stating the number of horizontal pixels the screen is displaying by vertical number (i.e. 800x600 or 1024x768). Believe it or not, this resolution stuff matters to your users.
Fewer pixels on a screen usually means the image viewed is larger (magnified). That's why making an image bigger in your image editor makes it look grainy. It has to use more pixels and may not know how to color them. It has to guess which colors to fill the magnified space with.
Not all computers are the same, right? So, not users have the same monitors. Also, they don't all have the same eyesight and their monitors may be at varied settings. That means we have to design for the lowest common denominator of user who we want to cater to.
Lately, the standard has swung to a 1024x768 resolution. That means that if a user with poor eyesight has to use 800x600 to view that site they must scroll both up and down and side to side to read the content. It is annoying for them. However, that group is in the minority.
To check which setting your monitor is on right click on your computer desktop and then "Properties". Click the "Settings" tab. You will see a slider for "Screen Resolution". Moving that one way or another will change the setting once you click "OK" on the bottom. We suggest considering changing your setting to 1024x768 if you are not there already and view your web site. How does it look?