The prefix property allows you to encode different symbols of data with variable-length without using any special markers in between. Any encoding algorithm that can represent different symbols without using common prefixes follows this property. Fixed-length codes inherently abide by this property.
An example of an encoding that doesn’t have the prefix property is this: You can see that there is ambiguity when decoding - the encoding for A prefixes the encoding for B. An example of an encoding that does have the prefix property is this: There is no ambiguity when decoding