Back in 1999 a lot of websites would consist of a full-page Adobe Flash program (something that no longer works in modern browsers) a bit like how a lot of websites now are basically JavaScript apps. Because the speed of internet connections was so low back then it could take a long time for those flash apps to load all their content, so while you were waiting they might play a little animation or something that served as an "intro".
Zombo com was a parody of those websites. There was no actual content, it just played a really really long intro that consisted of colorful blinking circles and audio of a guy saying variations of "welcome to zombo com, you can do anything here, anything at all" over and over again.