It means they're guinea pigs. If you mean a specific kind of squealing noise, you could upload a video. My guess is that they're begging for treats, though, since they especially do it when you go near them.
Some pigs are just naturally more talkative than others. None of their bad noises can really be described as squealing except for when they squeal in pain, which obviously isn't what's happening here. One of my pigs "narrates" everything that happens to her, and will wheek (beg-squealing) for literally any object I am holding, even if it's one of her sisters, or like, a pair of pants I'm putting on. She just loves to yell. Nothing wrong with it.
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