What do they mean when they say it's a socialist country?

In a nutshell:

Capitalism is an economic system or ideology where the means of production are privately owned.

Communism is an economic system or ideology where the means of production are commonly owned by everyone.

Socialism is an economic system or ideology that is intermediate between capitalism and communism. It allows for a mix of public and private ownership of the means of production and/or for regulating privately owned means of production so that they contribute to the common good instead of at the expense of it.

In the broadest sense nearly every modern economic system could be considered socialist since nowhere is a pure form of capitalism or communism practiced. Since Karl Marx saw socialism as a step toward communism many capitalist ideologues view any government participation in the economy as an irreversible slippery slope to totalitarian communism.

In the scheme of things the US is one of the least socialized nations in the world. This does not stop doctrinaire capitalist ideologues from branding anything even one tick to the left of some imagined libertarian ideal as socialism.