Tourists show up after a move has made headlines and depart after the first serious drawdown. The term is dismissive and describes a genuine cycle: participation peaks late, and the newest entrants hold the smallest cushion.
Everyone is a tourist somewhere. The useful version of the idea is self-directed: if you are trading an instrument you first heard about this month, with no measured edge in it, size like a visitor rather than like a resident. See herding and market-cycle-emotions.
Related: herding, market-cycle-emotions, dumb-money, bull-market-genius