Java is the main language used to develop Android apps. Most users don't need to worry about it. If your question is about development, please ask on our sister site Stack Overflow instead.
Developers can use the Java programming language to write Android apps. It's possible to use other languages too, but since all of the standard GUI components are written in Java, that's the most popular. 'Java' can also refer to the Java Virtual Machine (JVM), which is the foundation for running apps that were written in Java (or some other languages). Android has a special kind of JVM named dalvik. In general, each app is isolated in its own copy of Dalvik for security.
Although programming questions are off-topic for this site, power users and those curious about how Android works sometimes have questions that relate specifically to the java language or the dalvik JVM.