If you think GNU Backgammon plays too strong for you, you can add some noise to its evaluation. The number you add into the field in the evaluations setting is the limit amount of noise that will be added to the evaluation.
If noise is added to the evaluations GNU Backgammon will take a Box Müller transform of a point in the unit circle and add to each possible outcome probability. This means that the addition is random, but distributed so that it's more likely to have an noise addition close to zero than a noise addition close to the limit. The noise addition is limited to the number you put into the noise field in the dialog box.
If you check the box Deterministic noise, the noise added to each evaluation will be based on a sum of the bytes in the hash of the board position, which (by the central limit theorem) should have a normal distribution. In that way you will always have that same noise amount to a position, since the noise added to the evaluation is only depending on the position itself.
If you want GNU Backgammon to evaluate and play as strong as possible, you should not add any noise.