The uncertainty in a measured value is the possible variation in a measured value arising due to inaccuracies in the instrument being used. In the case described, the beats are 127 per minute, and the uncertainty is +/-4 beats. This means that the beats may actually be between 123 and 131.
The fractional uncertainty is: 4/127 = 0.031
And the percentage uncertainty is: 4/127 * 100 = 3.1%
