Interpreting the Jackstraw plot of Seurat

Question

[rohitsatyam102]

I am having a hard time understanding the Jackstraw plot and how it is made. A discussion on GitHub here and on StackExchange here give some insights but I still have a few questions:

1. What does the axis of the Jackstraw plot represent. What is the meaning of Theoretical [runif(1000)] and empirical.
2. What does the slope of the Jackstraw plot represents? What does each dot on that slope represent?
3. Is there a detailed yet simplistic step by step explanation on how the P values are calculated starting from PCA loadings?