In a sentence it is life not happening in one's way. In other words when there is a mismatch between "what is" and "what should be". The mismatch between what life offers as reality and what you believe and expect life should be.
These "what should be" are hearsay ideas, opinions and beliefs that the mind is carrying without questioning and validating deep enough. Every disturbance is the mind's way of reminding that the understanding is not correct and need to set it right. That the perception is biased and unclear.
Ignoring these doubts and inner alarms is not an option because a distraction is just short lived and it would come back to nag you. Pay attention, listen to it and course correct by dropping beliefs that can't stand truth, logic and sanity.
That's the only way to free your mind of the inner noises. When the mind is clear there is no doubt possible and no belief is needed because the mind now "knows".