The arabic word “Yaqeen” (یقین) means certainty, it refers to a level of surety that means there is no possibility of doubt without ones mind.
The opposite of certainty is doubt, or guesswork, in arabic “thann” ( الظن ), it is generally seen as a negative position, one where some one tries to guess or assume and not use proven techniques to obtain truth.
In matters of religion, such as in Fiqh, yaqeen is required, it is not enough to think in terms of probabilities or chance, therefore all methodologies used to generate Fiqh (usul al Fiqh) must also be able to yield certainty, but what exactly does that mean? Is certainly simply your best effort? or is it when the truth is reached?
In order to be completely certain about an issue, it would require complete knowledge of the subject at hand, however who can claim to have complete knowledge of any subject? History is the best witness to the continuous evolution of understanding of the world around us, and if there is only one thing that is true, its that what we know now is subject to refinement. That being the case, how can any laws be derived? Even if a purely logical and deductive approach is utilised from sound authentic sources, there is still a possibility of error, misinterpretation, or lack of available knowledge to make the correct conclusions. This does not make the position of the learned scholar the same as the guesswork of a layperson, as the layperson by definition has less knowledge than the learned person, so their answer has to be less certain, which brings us to the conclusion that not all certainties are equal.
Imagine an adult having a discussion with a child, it is relatively simply for the child to be wrong about an issue, however from their perspective they can still be absolutely certain about it, even though the adult knows they are wrong. So the individuals own feeling of how certain, or how “sensible” any position feels to them is completely irrelevant, in the same way that a child can be certain of Father Christmas because they heard a bang in the night and received presents in the morning, to them the evidence is clear and there is no doubt, or even reason to doubt. The logical approach then is to follow the person who has the soundest sources and most logical approach, and that scholar’s certainty is the greatest, not in terms of how sure they are, but in terms of the quality of their deductions.
In terms of religion, it would be very difficult to argue that you took the view of a layperson over that of a scholar thinking they are equal, or that it does not matter, that aspect is proven, it matters and the opinions are not equal. However, lets say a layperson did not know if something was allowed or not, and used their own judgement, which later turned out to be wrong, Allah could question why they did not take advice from someone more learned. However if a lay person found a recognised scholar, who is the most knowledgeable, and relied on their opinion, whether the scholar is right or wrong, what more could the lay person have done? They would have reached certainty that they could have done no more in their responsibility to God, and therefore they would have yaqeen.