What does "asp.net" mean?

ASP.NET pages aren't static; they have executable, server-side code behind them that may dynamically generate page content or interact with the serving system—they're active (web) server pages.

As an aside, ASP.NET isn't really an upgrade for ASP, it was just named that way to get it past corporate departments - you could sell it to your higher-ups as an "upgrade" to ASP even though it was pretty much a separate technology.

According to Microsoft, it refers to the ActiveX components.

Microsoft® Active Server Pages (ASP) is the server-side execution environment in Microsoft Internet Information Server (IIS) 3.0 that enables you to run ActiveX™ scripts and ActiveX server components on the server. By combining scripts and components, developers can create dynamic content and powerful Web-based applications easily.