After having a lot of contemplation on the spiritual matters, Lord Buddha came to a conclusion that everything in this physical world is marked by three characteristics, known as the Dharma Seals or the three characteristics of existence or Ti-Lakkhana in Pali. These three marks of conditioned existence are Anatta, Anicca and Dukkha.


Anatta
Anatta (a Pali word) or Anatman (a Sanskrit word) is basically the concept of a self or Atman or soul. This concept refers to an unchanging, permanent and static essence conceived by the virtue of existence. But the Buddha rejected the permanent concept of Atman and its relation with Brahma, the Vedantic monistic ideal. Instead, He emphasised on the changing character of the soul and preached that all the concepts of a substantial self were not correct and formed because of ignorance.
Anicca
Anicca or Anitva means that nothing is constant. All the things, feelings and experiences are inconsistent and impermanent. There is no such thing that lasts forever.
Dukkha
When we fail to grasp the first two conditions truly, we suffer and that suffering is known as Dukkha. We always crave for permanent satisfaction, but forget that nothing lasts not even satisfaction. Therefore, it is all about realisation, which can prevent us from Dukkha and not only Dukkha, but other two marks and their sufferings can also be eliminated by driving away ignorance.