What do you not understand? As the manual says, smoothed objects are the
expected value of variables and shocks given
the information available in all observations up to the final date (E_T (y_t)).
They use the full sample information for inference.
In contrast, the updated variables
contains the estimation of the expected value of
variables given the information available at the current date (E_t (y_t)).
They are basically real-time estimates as they only use information up to the current date t.
Most of the time, people are interested in smoothed values. You want to know the output gap in the past, using all available information to estimate it.