First off, as "webOS" implies, apps written for the platform are web-based -- HTML, JavaScript, that sort of stuff -- but unlike Apple's original vision for the iPhone, Palm's going to include libraries that allow devs to tap in to the Pre's hardware capabilities and interact closely with services exposed by the operating system.
Compared to previous versions of HTML, HTML5 makes it easier to create feature-rich Web-based applications that can be updated remotely with new functionality without requiring users to download and install an update each time.