As far as I know dump means drop, decrease or low and pump means increase or high so its better to buy if the token is dump and time to sell if the token is pump.
You are perfectly right actually when in market more people use to sell coin and the price will get down because of high supply while low demand it calls as dump of price in this situation we should buy more and never sell. On the other hand when more people use to buy bitcoin and get more bitcoin to save for long time, it makes the price high because it can make demand higher than supply in this situation we should take advantage and sell our coin.