This summary is not the rule. The rule text controls.
If someone is on trial for conspiracy, prosecutors usually have to show more than just an agreement to commit a crime. In many conspiracy cases, the law requires proof that someone actually took a real step to carry out the plan. This step is called an "overt act."
This rule says the overt act has to be spelled out in the written charges against the defendant, whether that's an indictment or an information. The prosecutor cannot convict someone based on some overt act that was never mentioned in the charges. At least one of the overt acts listed in the charging document must be proven at trial for a conviction to stand.
However, the rule also allows prosecutors to bring up other overt acts during the trial, even if those specific acts were not written into the charges. So while at least one charged act must be proven, the prosecution isn't limited to only talking about the acts listed in the paperwork. They can use additional evidence of other actions to help prove the conspiracy happened.