Sounds like a complicated math theorem. It's actually a good term for what happens inside of a big trend. Epidemics are an example of geometric progression. Starting with only a few people an infectious disease can spread in the population getting bigger and bigger. When a virus spreads it doubles and doubles and doubles.
Market trends are no different.
From nowhere violent market trends either up or down can appear. These trends often feed upon themselves and can quickly progress geometrically allowing the opportunity for huge profits if you were in early set with a trading plan. Human beings can have a hard time with this type of progression, because the end result often seems out of proportion to the cause. In order to appreciate why market epidemics can be so powerfully rewarding you must avoid the expectation of proportionality. You need to prepare for the possibility that sometimes big market changes follow small events, and that sometimes that change can happen very quickly.