Apache ActiveMQ is an open-source messaging and integration pattern server that allows for message throttling, redelivery, and delay. This document discusses how to install and configure ActiveMQ, including setting up dead letter queues and clustering multiple ActiveMQ instances. The key steps are: 1) Installing ActiveMQ on each node, 2) Configuring dead letter queues by setting redelivery policies in activemq.xml, and 3) Configuring clustering by giving each broker a unique name, connecting them to a shared SQL database, and starting one as the master node.