A Tourism Boom is a good situation in which a nation becomes particularly attractive to visitors from other countries, who will spend money in the nation and raise its GDP.
High Foreign Relations is the most important aspect of attracting tourists, but lenient Border Controls also play a part; any control over 20% discourages visitors while anything under it encourages them. Any form of Airline Tax will also discourage tourism.
In addition, Crime (especially of the violent sort), Antisocial Behavior, and the presence of Street Gangs all make a nation less attractive to tourists and the outbreak of a Contagious Disease keeps a nation too quarantined to have tourism at all.