Newbie Code question

price.nil? should not return true if the value of price is 0. nil
is an object that is the anti-matter of objects. It is something
that isn’t really there. It’s not 0, it’s not a blank string,
they would be something. No, this is nothing. ( does
anyone have a racing snail? ) The nil? method, available on all
objects, will only return true if the return value is nil ( ie nothing )

So the logic of the code says

add an error to the price field unless price is nil (nothing) or if price is greater than 0.

This means that price can be nil (or blank or however you want to think
about it but blank is another method in rails) or it can be
positive. Any other value and the error gets added to the price

Hope that helps a bit.