The first seal, that All Composite Products are Impermanent, refers to anything that is composed as impermanent. Understanding this seal will help us accept loss, ageing and death, which are all part of life, a composite product.
The second seal, All that is Contaminated is Suffering, refers to the nature of actions, emotions and thoughts contaminated by selfish attachment, aversion and ignorance, due to stained or contaminated actions, emotions and thoughts we become attracted to, or repulsed by.
The third seal, Aggregates are Devoid of a 'Self', is related to the teaching that speaks of everything as lacking inherent or intrinsic existence. Both person and phenomena should be judged objectively as they are, without mental projections, rather than holding on to the opinions of 'I have no self' or 'I have self'.