A pump and dump is when an individual with a lot of cash puts it into a small alt coin or company and that skyrockets the price. They then proceed to try and convince others to buy in and sell it at a peak. This then creates a herd mentality as the coin or stock loses in price creating a pumped up currency not really worth that value to drop. A few times it's insiders with more information about the news others it's just news dissemination.