Reactors are community spaces run by Microsoft where technology professionals can learn, connect, and discuss ideas. They provide workshops, panels and events on topics like machine learning, AI, blockchain, and cloud development. Attendees can learn new skills, hear from experts, and find resources from Microsoft. The document then describes an agenda for a reactor event on blockchain that includes introductions to Ethereum, Solidity, blockchain ideas, the Azure Blockchain Development Kit, and building a sample Blockchain application called Blockopoly.