When you sit down to practice Vipassana meditation, you observe your moment to moment experience with the intention of seeing the three characteristics: anicca (impermanence, change), dukkha (unsatisfactoriness, suffering) and anatta (not-self).
More accurately, you are tuning into the 3Cs, as they are always already the case. This is not a philosophical exercise – the practice is to stay at the immediate sensate level of your experience, with a degree of mental calm that allows you to observe manifesting reality without getting caught up in it.