Retrospective means that before the promulgation of the hong kong version of the national security law, if a hong kong person did something involving national security, he may also be held accountable. Since I don't know how the hong kong version of the national security law will be written, this is also speculation, but the worry is quite realistic.
In general international legal ghost mannequin effect provisions, non-retroactivity is generally a major principle. In hong kong, there is generally no retrospective effect before the promulgation of the law, and the law does not violate the law without express, but there are a few exceptions (such as the smuggling law). Article 12 of the chinese criminal law also stipulates that the criminal law has no retrospective force. But the national security law is likely to be different.
As we all know, china enacted the hong kong version of the national security law to deal with people who have always been anti-ccp in the eyes of china, such as jimmy lai of apple daily, zhuming li of the democratic party, professor tai yaoting of hong kong university, and huang zhifeng of hong kong zhongzhi, etc. Assuming they do nothing after the legislation, isn't china unable to deal with them?