The true proportion, in terms of statistics, is expressed as a fraction of the total set of data presented in which an activity, x, was performed.
If you observed or surveyed a person at random times engaged in various activities, then the true proportion would be the real amount of time spent on x activity as opposed to the time suggested by the more general terms of the sample. The true proportion is then calculated by means of a algebraic formula.
To learn more, have a look at previous Blurtit answers to such questions. Try Which One Of The Following Statements Expresses A True Proportion? or How Often You Should Collect Statistics For A Table?
You could also watch this film, which gives a basic introduction to the world of mathematical statistics: