So this is a realllly dumb dumb question... but I'll ask anyways, what the heck can I do with this? Build an app, yes, got it, but... what? Like if you said build a wordpress site I'd get it, but what kind of app can I build? to do what? And that app runs on what?
Looks like you can build anything that you could build with java/ruby/node code. And they would launch as many servers as you needed given the amount of traffic that it got. And you'd put the data into a nearby managed database like... Mongo (ew) or Postgres. Not listed: what operating system is running down there (going to guess Linux, since I don't know what building various binary ruby / node modules would actually look like on AIX :P)
It's similar to other Platform as a Service options. You build the app you want. The benefit is that you don't have to worry about setting up and scaling the architecture. For example you could deploy WordPress on AppFog PaaS. I don't think bluemix officially support PHP but PHP is there. Blue mix supports node or rails. so basically any node or rails app you wanted to build could be hosted.