The other answers have covered the meaning of Socialism in the Soviet doctrine (but see also What is 'real socialism'?). I address here the meaning of Socialism at the time of the 3rd International, and why it tried to promote a more revolutionary stance.
TL;DR: To save Marxist version of Socialism (vis-à-vis reformism/revisionism/parliamentarism/ministerialism) and to help the young Soviet Republic.
Pre-Marxist and Non-Marxist Socialism/Communism
The meaning of terms Socialism and Communism has evolved over time and in different political realities. Both notions existed before Marx came to stage, associated with Henri Saint-Simon, Charles Fourier, and Robert Owen (see History of Socialism.) Marx himself adopted Socialist/Communist view first under the influence of Moses Hess, sometimes referred to as the first German Communist, and later during his stay in Paris. Then again he was not the only one calling himself Communist - the other influential ideologues were Blanqui, Proudhon, Lassalle, Bakunin (anarchist-Communist) and others, with whom Marx often was in conflict, as evidenced by his various critical writings and events like demise of the 1st International (after a dispute with Bakunin.)
Socialism and Communism part their ways
Marx has developed his own view of Socialism/Communism, and was successful in popularizing it, which is why nowadays we mostly associate Communism with his name. During his lifetime he largely used the two terms interchangeably, and firm association of Marxism with Communism emerged mostly after the Russian October Revolution and massive propaganda by Comintern. As Karl Kautsky writes in Dictatorship of the proletariat:
They did away with the democratic institutions which had been
conquered by the Russian people in. the March Revolution.
Quite properly the Bolsheviks ceased to call themselves
Social Democrats, and described themselves as Communists.
Kautsky represents a practical stream, which was developing in Europe - notably in France, England, and Germany - while Marx was engaged in mostly theoretical work. The Socialist parties in this countries, although often sharing Marxist views, were scoring serious achievements via parliamentary engagement - which was contrary to Marxist view that opposed participating in bourgeois politics and considered revolution (complete breaking of the existing social and economical relationships) as the only way forward. This conflict surfaced on multiple occasions - in Marx harsh Critique of the Gotha program, when the newly born German Social-Democratic Party essentially adopted Lassalean view on State socialism, in Revisionist debate that eventually led to the split of the SDP into Socialist and more extreme Communist parties, in demise of the 1st International over the debate of Marx support for the violence by the French Commune (aka Blanquism) in French Ministerialism towards the end of the century, exemplified by Jean Jaures.
Democracy vs. Terror
The 3rd International emerged after the first proletarian revolution took place in Russia (The October Revolution of 1917). The Marxist view held that the revolution in a backward country could survive only if accompanied by simultaneous revolutions in more industrialized western countries. For the reasons described above, the Western Socialists were reluctant to unleash violence, as democratic political engagement bore fruits. Moreover, as their support for the war credits in 1914 has shown, they valued progress on the national level more than abstract Marxist internationalism. Being essentially a Moscow tool, the 3rd international tried to promote the more extreme position. This is reflected in the famous Terrorism and Communism debate between "the Pope of Communism" Kautsky (favoring democracy) and Lenin and Trotsky promoting the revolutionary terror (trivia: it was during this debate that Lenin coined label renegade Kautsky, which entered Soviet folklore.)
The term Socialism in its Marxist meaning is manifested in the constitutions of the USSR and its satellites. On the other hand, western states casually referred to as "Socialist" in fact describe themselves as Social Democracies or Democratic and Social states, whereas their economic system can be described as Welfare Capitalism.