Complex sentence: The estimator is a function of the sample of "n" observations drawn without observational bias from the whole sample space, in this example that sample would be the set of all measurements of yesterday's rainfall in all available rain gauges within the geography of interest.
Split into simpler sentences: The estimator is a function of the sample of "n" observations drawn without observational bias from the whole population of potential observations. In this example that sample would be the set of actual measurements of yesterday's rainfall from available rain gauges within the geography of interest.