This basic Theravada teaching describes the nature of existence — not as a physicist would (varying forms of matter/energy), but as a philosopher would. Everything arises, sticks around for a while, and then disappears — that’s impermanence (anicca). All experience is marked by suffering (dukkha) — this is the first noble truth. And all those suffering, impermanent things actually aren’t really things, that is, they have no self-nature. The three marks of existence counters the common human delusions that we aren’t really going to disappear, that suffering is a cosmic mistake, and that we have some kind of permanent essence. Mahayana teaching adds an even more fundamental aspect of existence: emptiness (sunyata).
For more information, see https://en.wikipedia.org/wiki/Three_marks_of_existence