Concurrency in Java refers to the ability of a Java program to execute multiple tasks or processes simultaneously. In other words, concurrency is the ability of a program to run multiple parts of the code concurrently, so that they can complete their execution simultaneously, and thus save time.

Java provides built-in support for concurrency through its multi-threading capabilities. A thread in Java is a lightweight process that can run concurrently with other threads in the same program. Java's thread model allows multiple threads to execute in parallel, sharing the same resources of the process.

Java's concurrency model allows developers to create complex programs that can perform multiple tasks simultaneously. Some of the common use cases of Java's concurrency model include server-side applications, web applications, network programming, and multimedia applications, where multiple tasks can be performed simultaneously.

Java's concurrency model provides several classes and interfaces to support multi-threading, such as Thread, Runnable, Callable, Executor, ExecutorService, and Future. These classes and interfaces provide mechanisms to create and manage threads, coordinate thread execution, share resources among threads, and handle synchronization and inter-thread communication.

However, writing concurrent programs in Java can be challenging, as concurrency can introduce several issues, such as race conditions, deadlocks, and thread interference. Proper synchronization and thread-safe programming practices are required to avoid these issues.