Actual five year old explanation:

Some people, who bake a birthday cake with their mom and dad, want the entire cake for themselves. If mom or dad take a piece anyway, they think that's not "sharing" but "stealing". Even though mom and dad helped bake the cake too. They think cake being taken from them is socialism.

long form:

1. When "socialism" started, it started in a BLOODY way. I mean, the Russian revolution, before that it was mostly just theory, but still "the poors" doing it, so that was ew for history writers. The Russian revolution itself did achieve some kind of no longer aristocratic system, but for many many people, the outcomes were not positive. There were some positive outcomes on average, literacy and food supply improved a lot over time. But we're talking about the negatives here. Then the whole stuff Stalin did and being in power for decades in a system that's supposed to be democratic didn't exactly improve the reputation. Don't forget that for a long time, world domination was the literal declared goal of the international communist party. They were legit "coming for you" because they were coming for everyone. So that's one whole topic covered.
2. Have you ever worked in a group project and someone didn't pull their weight? "The" argument against socialism is that that is going to be everyone. And then nothing gets done. But because society still does need some things to function, like food supply, electricity, etc. society will collapse, because nobody will do what's necessary. Because the mindset of people against socialism, is that external reward is always necessary for people to do things. If everyone has the same, unconditionally, there is no reward and no punishment. To finance the system anyway, the fear is that "socialism" would just tax everything or seize your property and redistribute it.

So when someone says "that's socialism", they fear that they will be robbed or killed or at least threatened. And to be fair, the thought of being robbed or harmed is scary.