Clarification requested on my original question - What do the colon and number 4 represent in the following argument - loglog(f,a​bs(p(:,4))​),grid;?

Clarification requested on my original question - What do the colon and number 4 represent in the following argument - loglog(f,abs(p(:,4))),grid;?

The original answer provided by Sean de Wolski (thanks for your answer) was: all rows (:) in the fourth (4) column of p.

Meanwhile, the lines of code preceding the loglog(...) were as given herein, below, wherefore, how do you know that "p" has dimension"4"?

sim('focbls',[0 2]); [p,f]=spectrum(posin,posfb,1024,0,[],1000); figure(1) clf; subplot(211); loglog(f,abs(p(:,4))),grid;