A metapage is a webpage that consists of embedded and connected webpages.
An example metapage showing two metaframes, one generating randome data, the other plotting whatever date it gets:
The two pages above are completely separate websites, and have no idea about the other. One simply passes data to the other, controlled by the metapage code running in the parent page.
A website can crunch data, show interactive plots and graphics, allow sophisticated user interaction. In other words, all manner of useful things.
When you have somethine like a website that is specialized and does something well, eventually people want to combine useful pieces into a bigger whole.
These are metapages.
See the docs for more detailed documentation.