very very confuse

[0ManishKarki 0]

hi there i am very stuck with this problem   i know pos.top has value in it, i dont uderstand pos.top - 30 i know its substracting but i dont understand it but i dont understand .css('top ', pos.top -30 + 'px') , 

where this is from jquery

var pas = $(this).position();

$('div.tooltip')

       .css('top', pos.top - 30 +'px')

       .css('left', pos.left + 'px');

[administrator 435]

This comes down to basic CSS where you can move an element in a + direction or - ... negative direction on the X or Y axis. 

[0ManishKarki 0]

good morning sir , i repeat the video more than 5 times its very difficult  for me to understand can you solve my problem with example in simple english.