RICO stands for Racketeer Influenced Corrupt Organization. Very, very importantly: it makes it a felony, punishable by a double-digit sentence, to conspire to commit a crime via an organized crime operation, or something that appears to be a "racket." I emphasize "conspire" because that's the key. In order for a defendant to be convicted of a felony, the prosecution usually requires a "corroborating witness" to back up the testimony of a main witness. In a conspiracy case, no corroborating witness is required to get a conviction. As you know, it's very difficult to get anyone to testify against a gangster--much less to get two people to testify. That's why so many mobsters got away with murder, literally. But under RICO, all they need is one witness, plus some other kind of circumstantial evidence, such as surveillance tapes or phone taps. If you put it in the context of the Godfather granting Bonasera's wish: Vito gave orders only to Hagen, who passed them on to Clemenza, who passed them on to Paulie. Now, if Hagen suddenly decided to turn rat, without RICO, it'd be his word against Vito's, and Vito would get off. But with RICO, all it'd take to convict Vito would be Hagen's testimony and one other piece of evidence, such as a bug on Vito's phone if Bonasera called to thank him for "granting my wish."
Not only that, RICO applies to victims of organized crime. Let's say I'm a degenerate gambler and I owe big bucks to a Mob loan shark. He forces me to "bust out" my business to pay him off. I can be prosecuted under RICO for operating a "racketeer influenced organization," and I could face a double-digit sentence. This provides me with a huge incentive to testify against the Mob guy.