It means visitors coming from IP addresses associated with a ".com" domain.
As Silvino Ferreira Jr points out, this does not map directly to users in the US. Many ".com" domains exist around the world, including a great many hosting providers.
Conversely, there are several companies and services using country-specific domain TLD extensions located in the US, or other unaffiliated country. Bit.ly, for example, is a company based in the US, with a country code domain TLD from Libya (.ly).
The only cheap and easy way to determine the country-level breakdown of visitors to a site is to use their IP address, rather than their source domain. This is still potentially inaccurate, but it's much easier for geolocation data tracking companies to correct associations of blocks of assigned IPs back to specific countries, regions, and cities, than it is to do so for each individual domain name on the Internet.
A few links to GeoIP information providers:
- ip2location.com
- maxmind.com
- google.com/analytics (if you're not using this for your website stats, it will provide geolocation info, as well as many other useful demographic tools for studying your site visitors)
One more thing: depending on the reason you're targeting the geographic location of visitors, you may experience less than ideal outcomes, even with highly-accurate information: http://xkcd.com/713/
deano.com 
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