HUB technically means a common point where lot of other things can connect, for example lot of computers connected over a network. Now when it comes to Windows Phone 7, Hub forms a unique feature and here it means that an Application can extend beyond what you can see on the screen.
For example lets say if you have an Album which displays pictures from Flickr, Windows Live Gallery, Facebook etc with latest one being displayed when you launch it. Now if it’s a standard app probably it will display in thumbnails and you click next to see more but with Hub you can flick right or to left to find more. Doesn’t sound different ?
Imagine a huge wallpaper on the wall with loads of picture but you get only small screen to see it. You cannot see it all but you can move right or left to see it. This is what hub means here. It breaks the barrier of apps to be developed with fixed screen size. Below is what Picture Hub in Windows Phone 7 Looks like :
Hubs in Windows Phone 7 are the universal concept. You get HUB for every thing right from People, Games to Marketplace.