I have few questions about the plot:
i) What are we plotting along y-axis? Is it the probability density? ii) What does the width of the plot of individual parton tell me? What I mean is, let's say for $Q^2 = 10$, the width of s is very thick for a very small value of x and when $Q^2 = 10^4$, it becomes very sharp. What does it tell me? iii) Also, is it because this is a p-p collision, and thus, the plot of u has its peak at almost twice the height as that of d? Does this particular value of x give me any physical significance?
P.S. I did check the website for similar kind of questions, and there were some good answers, but I could not find the one that suits to my level of understanding. Any reference to a useful material would be helpful.