Before the war, the USSR gave Finland several proposals to create mutually benefical alliances to defend both of them from inevitable Nazi aggression, but in every case Finland refused.

“Our proposals in the negotiations with Finland are modest, and they are confined to the minimum, short of which is it impossible to ensure the safety of the Soviet Union”
[...]
“The Government of Finland feels obliged to maintain the attitude which it has taken up from the outset regarding the proposal that it should lease the port of Hanko and the surrounding district to the Government of the USSR and place the Bay of Lappohja at the disposal of the naval forces of the USSR for use as an anchorage”.

—V. M. Molotov: The Development of Finnish-Soviet Relations

The Soviet Union needed strategic security from the Nazis. They were negotiating to rent some small pieces of land to be able to close the Gulf of Finland to enemy naval forces in the event of war to protect Leningrad. The USSR also needed the border near Leningrad itself to be moved outside of the artillery range. In exchange for a total of 2700km² the Soviets offered 5500km².

The Nazi-sympathizing government of Finland of course refused and took a hostile stance against the USSR. Eventually there was a build up of troops at the border near Leningrad and shells were fired into Soviet territory. This led to a costly war that the USSR eventually won and in peace demanded essentially what they had asked for before the war.

The USSR managed to take several important regions of Finland. The areas taken contained several large cities and industrial areas and ports, giving them greater access to the gulf.

“The meaning of the war in Finland lay in the necessity for safeguarding the security of the north-western frontiers of the Soviet Union and above all of the safeguarding of the security of Leningrad.

The Soviet Union smashed the Finnish Army and having every opportunity of occupying the whole of Finland, did not do so and did not demand any indemnities for her war expenses, as any other power would have done, but confined her demands to the minimum.

We pursued no other object in the Peace Treaty but that of safeguarding the security of Leningrad, Murmansk and the Murmansk railway”.

—V. M. Molotov: Speech to the 6th Session of the Supreme Soviet of the USSR

In this sense, the USSR won in that they got what they wanted (achieved their war aims). The fact that it took the Red Army several months to defeat the Finnish forces was used as propaganda to claim that the Red Army was ineffective.

adapted from a Quora post by David King