This summary is not the rule. The rule text controls.
If someone gets hurt or something goes wrong, and afterward a person or company fixes the problem or adds a safety measure, that fix normally cannot be used as evidence in court to prove they were careless or at fault. For example, if a store adds a new warning sign after someone slips and falls, the injured person cannot point to that new sign as proof the store was negligent before the accident.
The idea behind this rule is to encourage people to make things safer without worrying that doing so will be used against them in court. If companies knew that fixing a hazard could count as an admission of guilt, they might avoid making improvements at all. This rule removes that fear so people and businesses feel free to correct problems once they discover them.
It's worth knowing that this rule usually only blocks the evidence for proving fault. It does not always stop the evidence from being used for other purposes, like showing that a fix was possible or proving who owned or controlled something. Courts look at the specific reason someone wants to bring up the later fix before deciding if it can come in.