Socialism is a very broad set of ideas that essentially boil down to: value created by labor should be shared among those who create it, rather than a central owner who did not create it. The opposite of workers capturing the value of their own labor is called capitalism.
In most of the first world, the way socialism works is that everyone (including employers) pays taxes, which broadly fund a set of life-improving social services (like free healthcare) that effectively redistributes the capital generated by the economy to all people who live within it. These redistributions tend to reduce other dangers, like starving and dying of sickness.
In the United States, things are a little different: everyone pays taxes (although the largest employers and the richest people can mostly skip it) and the taxes fund some programs that benefit some people, but most of it goes to either the military or corporations. America has some socialist policies and programs in place, such as Social Security, Medicare, SNAP, the aggressive subsidization of the farming industry, and various housing initiatives, as well as some directly under the control of cities and states, but mostly doesn't acknowledge the socialist nature of these programs in public discourse. Instead, many people just use "socialism" as a slur without really knowing what it means—especially boomers.
Previously, America was a lot more socialist—many of the existing social programs that provide even the minimal safety net in place were created as part of President Roosevelt's enormously populare "New Deal," which saved the country and its economy after the collapse of the capitalist stock market in the late 1920s and early 1930s.
Socialism as a broader set of ideas prompted the labor movement in the late 1800s and early 1900s. The labor movement in turn generated perks like child labor laws, minimum wages, and the weekend. In Europe, workers' movements led to the adoption of the "Nordic model" in Scandinavia, essentially a multi-level set of social and economic policies that guarantee basic material security to everyone in the country. This arrangement is happily chugging along—four of the five Scandinavian countries took places 1-4 in the 2019 "World Happiness Index," although Sweden is in 7th place. For comparison, the United States came in 19th.
However, historically, socialism has been used as an excuse for despots and fascists to take power. The USSR, for example, started as a socialist experiment but quickly came under despotic control. Nobody really advocates for total state control of everything in that manner today (and we generally refer to total state control as communism instead), and the difference between violently overthrowing a government by conscripting peasants into an army (i.e. the Russian Civil War) and just like, making healthcare free (socialism today) is obviously incredibly vast. Separately, it's widely claimed that the Nazis were socialist somehow, since it was in their name—but their actual politics were fascist nationalism, which is pretty much the opposite of socialism, and their ideology was basically just extreme racism. They just used the word in their party name because socialism was really popular at the time.
Socialism in its extremely soft, currently-implemented-in-Europe version is increasingly popular among millennials and "Gen Z."
All in all, today's notions of socialism, which essentially just mean "spend tax money on social safety nets for the common good," are not very dangerous.
