A MicroKernel is a low-level subsystem that defines major architecture in a system without actually providing any specific architecture itself. For example, a MicroKernel may provide the generic notion of a Service or Configuration, but leave systems built on top it to provide how major features work.
You can think of a MicroKernel as being similar to a BIOS on most old PCs -- it allows the software to talk generically to the hardware underneath. Indeed, the MicroKernel was developed as a way of avoiding the feature-bloat that OS were moving towards; for example, NeXT used the Mach microkernel to build its Unix-based layer on top.
Building a system using a MicroKernel allows for easy decoupling between implementation and interface, because the MicroKernel provides a cleanly defined set of APIs that developers can use. Further, it is intended to be small, so that it doesn't provide everything, but just the basics on top of which a more fully featured Kernel can be built.
Avalon, HiveMind are examples of a MicroKernel architecture.