The impersonated token is the token whose picture will be displayed in the chat when a player is talking or executing a macro.
In general, the impersonated token will correspond to the token impersonated through the user interface. To impersonate a token this way, right click on it, and select the option "Impersonate". Alternatively, you can type the command
/im token_name to impersonate a token.
Some macros will temporarily impersonate another token for the duration of the macro. This is the case for macros with "Apply to Selected Tokens", enabled - for these macros, the impersonated token will temporary be the one selected on the map. Once the macro ends, the impersonated token will revert to what it was before the macro started.