With the aim to "provide insights into what has changed in response to policies aimed at combating Covid-19", Google's Community Mobility Reports join a list of companies that are sharing location information to aid authorities in the assessment of the rapidly developing coronavirus pandemic and subsequent restrictions on movement. As Google puts it, they are "to help you and public health officials understand responses to social distancing guidance related to Covid-19."
The reports are based on data from its users which have allowed the company to save their location history. As explained in the reports, these changes in behavior are calculated "using the same kind of aggregated and anonymized data used to show popular times for places in Google Maps."
As this infographic using data for the hard-hit U.S. state of New York shows, the lockdown measures there are indeed changing residents' movements. The largest reduction was observed in relation to transit station, for which there was a drop of 68 percent observed. Retail and recreation saw a similar fall of 62 percent, while residential unsurprisingly increased. As Google notes, the data represents a sample of its users and may not represent the exact behavior of the wider population.