Without quoting directly from the text give me a summary of what a restricted Boltzmann machine is?

A restricted Boltzmann machine (RBM) is a category of an artificial neural network invented by Paul Smolensky and popularized by Geoffrey Hinton. RBM's have been useful in various applications within the field of machine learning and can be trained in supervised and unsupervised ways.

RBMS is a restricted variant of Boltzmann machines that constrains their neurons; a pair of nodes from each of the two groups of neurons, visible and hidden forming a bipartite graph. Connection can only be between two groups and not within groups (visible and hidden). This constrained formulation results in the ability to more efficiently train algorithms. 

RBM's can also be integrated into deep learning networks such as deep belief networks with options to fine tune those networks using gradient descent and backpropagation.