Thread safety in Java refers to ensuring that a shared resource is accessed in a safe and synchronized way by multiple threads. In other words, when multiple threads are accessing a shared resource simultaneously, we need to make sure that they do not interfere with each other and cause unexpected behavior.

Java provides synchronization mechanisms like synchronized blocks and methods, volatile variables, and locks to ensure thread safety. These mechanisms prevent multiple threads from accessing the same shared resource at the same time, or they ensure that shared resources are accessed in a consistent and predictable manner.

To make a class thread-safe, we need to ensure that all its methods and fields are synchronized. For instance, a synchronized method can be accessed by only one thread at a time, while a volatile variable ensures that all threads can access the latest value of a variable.

It is essential to ensure thread safety because even small errors in synchronization can result in race conditions, deadlocks, and other issues that can cause unpredictable behavior in a program. Thread safety is, therefore, a crucial aspect of writing reliable and robust multithreaded applications.