Questions About GPS Measurements

[aholaday]

    Hello! I like to use Phyphox to record my walking and biking trips using the Location GPS app. 

When I export the data from a session, a rich set of data is provided. I have a couple questions about this data:

What exactly do Horizontal Accuracy and Vertical Accuracy mean / where do these values come from? 
It seems unrealistic to assume there is a guarantee that my actual position is within x meters of the measured position. My guess is the accuracy expresses an uncertainty that perhaps can be given by a distribution where, say, I can be 95% confident my actual position is within x meters of the measured position. Is that the case?

This leads to my second question:

How is Distance (km) data computed? 
A simple way would be to compute the straight-line distance between two (measured) positions. However, given the uncertainty in the measurement, I think this tend to overstate the actual distance traveled in the time delta t. Is any correction applied for an expected overstatement based on the accuracy values?

Thanks for looking at my questions!