Bohr's model of an atom
Think of the planetary system which inspired Bohr to think about his atomic model. The Earth is rotating around the Sun in a fixed orbit. What keeps the Earth rotating around the Sun? The mechanics of the planetary motion, and other electrical phenomena were very well understood in Bohr's time, so much so that by late 1880 to early 1900 a physics professor advised his student, who would become a Nobel laureate later, not to study physics because there is nothing left to do in physics

...here might still be a little dust or bubble at one or the other angle to check and classify, but the system as a whole is quite secure, and theoretical physics is noticeably closer to that degree of perfection that geometry has had for centuries..
"Max Planck: Wege zur Physikalischen Erkenntnis."

So classical electrodynamics (motion of charges) predicted in Bohr's time that a moving negatively charged body should spiral into the positively charged proton. Obviously this was not happening. Secondly, a moving charge should emit electromagnetic radiation because the great Maxwell had already established that.

Bohr had a moment of enlightenment. He just said let us say that the electron does not spiral into the nucleus but stays in a fixed orbit nor it emits radiation. Lo and behold, that assumption helped him explain most the hydrogen atomic spectrum in the UV, Visible, and in the near infrared. This was a big big feat! That is why when you read Bohr's theory, his assumptions are called Bohr's postulates.

What is a postulate in logic as per a dictionary's definition? "A fundamental principle, presupposition, or condition, esp. one assumed as the basis of a discipline or theory; (also) a proposition that is (or is claimed should be) taken as granted; esp. one (to be) used as a basis for reasoning or discussion, a premise."

"Taken for granted" is the keyword, which means when you are reading my theory, accept my assumptions that they are true-don't question them.