If this analogy/explanation helps:
"The pontifical secret was essentially the 'Top Secret' for the Vatican," says the National Catholic Reporter's Vatican correspondent, Joshua McElwee. "It was imposed officially in 1974 as a way of trying to protect the name of both the accuser and the accused until the point at which there had been a firm judgment." [...]
The changes to the "pontifical secret" rule mark a sharp departure from the previous policy. While the Vatican's new decree includes guidance about maintaining the integrity and confidentiality of church proceedings, it promises to quash what has been a common refrain from church authorities involved in abuse cases — that they were not authorized to share information with statutory authorities or victims.
Basically, the new Vatican rules seem to allow [more] extensive cooperation with civil authorities in these matters:
According to Italian jurist Giuseppe Dalla Torre, ex-president of the Vatican’s tribunal, the new changes “contribute to favoring a transition in canon law from an attitude of diffidence and self-defense regarding civil law, to an attitude of trust and healthy collaboration.” [...]
“When civil law establishes an obligation to report on the part of someone informed about the facts,” he said, “the reduction of the pontifical secret and the detail on the limits of the ‘office secret’ permit a ready fulfillment of the requirements of the law, thereby favoring full collaboration with civil authorities and preventing illegitimate incursions of the civil authority into the canonical sphere.”
Basically, prior to this change, the penalties for breaking the pontifical secret, which included excommunication, could have applied to someone from the Curia who informed civil authorities of allegations of sexual abuse, prior to their full resolution by a church tribunal.
Outside of Vatican proper, it is indeed the case that civil authorities had the means to get such information anyway, e.g. as NPR notes:
Boston, Mass., attorney Mitchell Garabedian — who has represented hundreds of abuse survivors — called the shift "a small step" toward transparency that could help victims as they try to heal. But he added that many law enforcement agencies likely already had the power to use subpoenas to obtain the same result.
Hower, as AP notes
But even under the threat of subpoenas and raids, bishops have sometimes felt compelled to withhold canonical proceedings given the “pontifical secret” rule, unless given permission to hand documents over by the Vatican. The new [church] law makes that explicit permission no longer required.
Do note (for example) the long-running saga of Australian archbishop Wilson, in this context.
As I understand it, the Vatican essentially has no extradition treaties for its own citizens, with some exceptions (under art 22 of the Lateran Treaty) for people who have "taken refuge" in Vatican territory and/or buildings and are liable under both Italian and Vatican criminal law. So, other than threatening them with arrest abroad (possibly in violation of diplomatic immunity arrangements) there is/was no way to compel top Vatican officials to comply with criminal investigations in other countries, including the release of information. For example, last year the Vatican refused a French summons for a Spanish cardinal to appear to testify in a French court (in the trial of a French cardinal).