In Buddhism, the Three Marks of Existence are things that all living beings have in common. They are impermanence, pain, and egolessness.
According to Buddhist doctrine, everything in life is impermanent. Even what we think of as our higher selves (our personalities and our reason) are constantly changing. This is where egolessness comes in. We do not have any permanent thing that we can truly call our "self." These facts cause us to experience pain because we are always trying to figure out our place in a world that is constantly changing. We are never able to achieve this and therefore we feel pain and dissatisfaction.
The Three Marks of Existence, then, are the negative things that define our existence in this world.