One event that triggers a dispersed series of similar events, that may yet again trigger even more events. While some reactions are linear, chain reactions can spread geometrically (aka snowballing), potentially causing large and unexpected impact.
For example, one bank failing in the Great Depression would set off failures at many other banks. One currency can quickly devalue, setting off a chain reaction of similar devaluations. Atomic bombs are the result of chain reactions, with energy being released from a few molecules releasing the energy in its neighbors. A single match can burn down a forest. Chain reactions require some type of distributed energy patter. If one wants to stop a chain reaction, one can exhaust the energy in a controlled way.
Some organizations go through chain reactions of key people leaving. Others get multiple subsequent bumps up or down in stock price. Reorganizations can cascade, as can new leadership and direction. Interest hikes, lowering or raising prices, spreading rumors, and viruses each have a chain reaction all of their own. Word of mouth about how good or bad a product, even formal learning program, is can spread like wildfire; when positive, it is called buzz or viral marketing, and can be helped along by a "tell a friend" button.