y-hat is our estimate of what the actual mean response (µy) is at given predictor values. b0 is our estimate (with our sample) of the actual y-intercept for the population. b1 is our estimate (with our sample) of the actual change in predicted mean response for each unit +/- in our predictor variable.
µy is the population's mean response to given predictor variable values. β0 is the actual y-intercept for the population's regression. β1 is the actual change in the population's mean response for each unit +/- in our predictor variable.
With confidence intervals, we can do our best to set an interval that we believe the true population parameters (β0, β1, βk) lie within at a certain level of confidence!