It stands in contrast to the closed or proprietary systems of Microsoft and Apple who restrict access to the core or sourcecode for their operating systems.
Android is open source software, which means developers who don't work for Google are allowed to see thesourcecode and create a range of applications to work with it.
And Microsoft used its Windows monopoly to help its applications division--the programmers who write software like Microsoft Word--by giving them preferred access to the complex Windows sourcecode.