Java is a high-level, object-oriented programming language used to build applications.
The important thing about Java is that when you write Java code, you generally don't write code specifically for Windows, Linux, or macOS. Java was designed so that the same Java program can run on different operating systems.
Java also has a huge standard library and ecosystem, which is why it's commonly used for backend applications, enterprise software, Android historically, APIs, distributed systems, etc.
When you write something like:
System.out.println("Hello");
you're writing Java source code. Your computer's CPU doesn't directly understand Java source code. There are steps between your Java code and the actual machine instructions that the CPU executes.
When people say "features of Java," they're basically talking about the characteristics that make Java useful.
Object-oriented means Java organizes programs around classes and objects. Concepts such as inheritance, polymorphism, abstraction, and encapsulation are central to Java.
Platform independent means the same compiled Java program can run on different operating systems as long as an appropriate JVM is available.
Robust means Java has features that help make programs more reliable, such as strong type checking, exception handling, and automatic memory management.
Secure means Java was designed with various security mechanisms, including bytecode verification and controlled runtime behavior.
Multithreaded means Java has built-in support for running multiple threads of execution concurrently.
Portable means Java programs can be moved between different environments with relatively little change.
Automatic memory management means Java has a Garbage Collector that manages memory occupied by objects that are no longer needed.
High performance means Java isn't interpreted in the simplest possible way forever. Modern JVMs use techniques such as JIT (Just-In-Time) compilation to convert frequently executed bytecode into native machine instructions at runtime.