What is a TLAB and what is it for?

Question

I recently used Java Mission Control (JMC) to track a memory Leak. In doing so I realized that JMC reports memory allocations inside and outside of Thread Local Allocation Buffers (TLAB). In a quick Google search I read that allocations of new objects within a TLAB is more efficient than outside.

But what exactly is a TLAB? How does it make allocating new objects more efficient? And what makes JVM choose to allocate memory inside or outside of a TLAB?