The sophists were wandering teachers of rhetoric and virtue who began showing up in Athens in the 5th century BCE. You could say that sophists were sort of like a cross between lawyers and university professors. But for whatever reason, they really got under Plato's skin. Never mind that Socrates himself was often accused of being a kind of sophist (that was, in effect, one of the charges against him in his trial)—the sophists' approach seemed to be a competitor to that of the philosophers'.
And that meant they were the enemy.
So we find Plato again and again returning to the theme of sophistry in his dialogues (a good number of Socrates' interlocutors are, in fact, sophists as well). And he continually beats up on the poor guys: the sophists are relativists, they mislead us, they are not lovers of truth.
Of course, being Plato, he can't resist muddying the waters. The real nature of sophistry is somehow very hard to define, and the portrait of the sophist that emerges in Plato's dialogue of that name sounds suspiciously like…Socrates.
Ah, Plato, can't you one time just have a nice old-fashioned fight with someone?