What is heterogeneity challenge in software engineering?

Computers are pervasive in modern life. When users perform transactions, they usually do not limit their access to their own computers. Instead, the users tend to access systems distributed globally across networks with heterogeneous technology and support systems. Further, legacy systems integrate with modern systems creating new challenges in building systems that are flexible. Heterogeneity challenge is the challenge involved in developing flexible systems that work well in different technological background.

Ref - Software Engineering, Ian Sommerville, 7th Ed, Addison Wesley