You would go out for example as a guy in a fedora and a suit to a specific place one day and you would do it again the next day with a completely different appearance in a different location. If something bad happens you can start over with a new persona and nobody would even know it was actually you. Kind of like dying in a video game and starting over as a different character but with slightly different intentions because you want to be careful you don't resemble the last persona that got caught and discovered. This is the best explanation I could come up with.
Generally inter-personally you can be whoever you want to be in a given moment. There are ways to change the appearance of your face with makeup, you can wear padding to create different body shapes, put lifts in your shoes to change your height.
Tequila_Wolf wrote
It'd help us if you gave a bit more information about what you meant and what you were doing it for.