PC sales only make up 20% of Microsoft's total revenue

It seems that most of Microsoft's profit comes from it's stronghold enterprise side. Welp, PCs "can die off", and the gods of technology will still make billions and billions.

http://www.reuters.com/article/2013/04/19/microsoft-results-licensing-idUSL2N0D52WB20130419?feedType=RSS

I don't understand this sentence though, can someone explain this to me?

The strength of that model is reflected in Microsoft's total unearned revenue balance - a measure of the strength of its long-term business - which rose 13 percent from a year earlier and now stands at $17.1 billion.
Looks like Microsoft will always get the last laugh. Har, har har.