It's basically a strategy for agrarian, predominantly pre-capitalist countries to skip through certain aspects of capitalist development before developing socialist institutions.
Take early 20th century and what you could do to it. On one hand, it could enjoy liberal-democratic freedoms and capitalism, develop itself like western countries did, and be modernized gradually through private initiative and bourgeois rights. The proletariat, a virtually inexisting social class, would grow like it did in the West (the capitalists, now free and acting with the help of the state, would create businesses and industries that are based on wage labour, leading to the growth of the wage worker's class) and then traditional conditions for Socialism would be available with time.
That's the standard Marxist line.
But there's another option. Instead of going through a long capitalist stage you can have a proletarian state that imports and manages all the would-be private industry from outside, that claims monopoly on those areas of the economy and that regulates trade between native producers and the outside world, and as a result you have a country being modernized, with a growing working class, and without bourgeois dominance (the native bourgeoisie remains weak and limited by design). Private initiative can still exist, and so can democratic freedoms, but they supplement this general purpose instead of being seen as a historical stage to aspire to.
The obstacle is, of course, that once you're done using the state to develop the country you've reached a ceiling. As an isolated, recently modernized state you can't beat the capitalist world in innovation and technology, you're always following their pace and a few steps behind. And of course, if you're a Worker's Stage, even if isolated, you can only get so powerful before you become a risk. That's why making the revolution international is as important as the political economy you choose. They complement each other.
This is more or less what it means (to Trotsky at least, it's first used by Marx and Engels in one particular context that I haven't studied, but the term is now mostly associated with him). It doesn't necessarily imply invading and forcing revolutions everywhere, because remember the battlefield between International Revolution and Socialism in One Country was the Comintern, not the national military doctrine. Usually that would not be a realistic strategy because foreign invasion is not the best way to persuade people to join you, and Trotsky himself only advocated Soviet aggression when that was done for defense against Fascism (like in Finland).