A workqueue is an abstraction layer over the creation and management of kernel worker threads. They help solve a crucial problem: directly working with kernel threads, especially when several are involved, is not only difficult but can quite easily result in dangerous bugs such as races (and thus the potential for deadlock), as well as poor thread management, resulting in efficiency losses. Workqueues are bottom-half mechanisms that are employed within the Linux kernel (along with tasklets and softirqs).
The modern workqueue implementation in the Linux kernel – called the concurrency managed work queue (cmwq) – is really a pretty elaborate framework, with various strategies for dynamically and efficiently provisioning kernel threads based on specific requirements.