Drive by Downloads are a common technique of criminal hackers to spread malicious software. The hackers infect another wise legitimate web site with malware that will download to a victim's computer when they visit the site or click on a link at the site.
Criminals use this technique to spread all sorts of mischief and mayhem, such as financial fraud or controlling user's computer to spread spam, virtues of launch denial of service attacks.
What's important for you to understand is that Comodo's containment technology ensures that any malicious software you download for any reason will not harm your computer. The download might hit its target, but Comodo ensures that the threat is contained and no harm is done. Network administrators should also understand the Comodo's containment technology is also the best bet to keep their servers free of such malware so they are not the host of drive by download.