Buddhahood is a state free of all obstructions to knowledge and disturbing emotions. It is the state in which the mind is fully evolved

Our teacher, Sakyamuni Buddha, is one among the thousand Buddhas of this aeon. These Buddhas were not Buddhas from the beginning, but were once sentient beings like ourselves. How they came to be Buddhas is this.

Of body and mind, mind is predominant, for body and speech are under the influence of the mind. Afflictions such as desire do not contaminate the nature of the mind, for the nature of the mind is pure, uncontaminated by any taint. Afflictions are peripheral factors of a mind, and through gradually transforming all types of defects, such as these afflictions, the adventitious taints can be completely removed. This state of complete purification is Buddhahood; therefore, Buddhists do not assert that there is any Buddha who has been enlightened from the beginning.
from 'The Buddhism of Tibet'

Only a Buddha has extinguished all faults and gained all attainments. Therefore, one should mentally go for refuge to a Buddha, praise him with speech, and respect him physically. One should enter the teaching of such a being.
A Buddha's abandonment of defects is of three types: good, complete, and irreversible. Good abandonment involves overcoming obstructions through their antidotes, not just through withdrawing from those activities. Complete abandonment is not trifling, forsaking only some afflictions or just the manifest afflictions, but forsaking all obstructions. Irreversible abandonment overcomes the seeds of afflictions and other obstructions in such a way that defects will never arise again, even when conditions favourable to them are present.
Tantra in Tibet

More than twenty-five hundred years ago, a man who had been searching for the truth for many, many lifetimes came to a quiet place in northern India and sat down under a tree. He continued to sit under the tree, with immense resolve, and vowed not to get up until he had found the truth.
At dusk, it is said, he conquered all the dark forces of delusion; and early the next morning, as the planet Venus broke in the dawn sky, the man was rewarded for his age-long patience, discipline, and flawless concentration by achieving the final goal of human existence: enlightenment.
At that sacred moment, the earth itself shuddered, as if “drunk with bliss,” and, as the scriptures tell us: “No one anywhere was angry, ill or sad; no one did evil, none was proud; the world became quite quiet, as though it had reached full perfection.” This man became known as Buddha.

On that momentous night when Buddha attained enlightenment, it is said that he went through several different stages of awakening. In the first, with his mind “collected and purified, without blemish, free of defilements, grown soft, workable, fixed and immovable,” he turned his attention to the recollection of his previous lives. This is what he tells us of that experience:

I remembered many, many former existences I had passed through: one, two births, three, four, five . . . fifty, one hundred . . . a hundred thousand, in various world-periods. I knew everything about these various births: where they had taken place, what my name had been, which family I had been born into, and what I had done. I lived through again the good and bad fortune of each life and my death in each life, and came to life again and again. In this way I recalled innumerable previous existences with their exact characteristic features and circumstances. This knowledge I gained in the first watch of the night.