count the height of a tree

I have a function that count the height of a tree

function countheight(root:tree):integer;
var left, right:integer;
begin
  if root=nil then countheight:=0
  else
     begin
   left:=countheight(root^.left);
   right:=countheight(root^.right);
   if left>right then countheight:=1+left
    else countheight:=1+right
   end;
end;

I don't know the IF condition so it can calculate the height of a tree
 
Please explain in details