So for example if two mates talk about that they are going to kill person Y, that is the agreement. And then one of them buys a gun, that is an overt act. And even if person Y has very good security, and the two dudes have no chance in hell of murdering Y ever, that is still a “conspiracy to muder” charge.
Probably most legal systems have something like that.
The interesting thing about US law - at least compared to German law (a Civil law country) - is the fact that even if you succeed with X, you can still be charged with conspiracy to commit X on top.
Conspiracy requires at least 2 people.
It's a way to let a prosecutor get ahead of an unsuccesful, and basically a freebie in terms of sentence amplification.
Now, a wiretap of a boss ordering a hit could result in the same sentence as the gunman.
Unfortunately it has been abused greatly, including drivers who never got out of the car sentenced to death for a 7/11 robbery gone bad, etc.
Further, as you can imagine, it gives the prosecution massive (many would say unfair) leverage over associates to a crime, given that they could face the same, full sentence if they don't cooperate. It's a major reason behind the enormous percentage of criminal charges not going to trial.
In the Alameda case this could make everyone in on the conspiracy liable to the same exposure as SBF