What do these expressions mean in general: first-level, second-level, ... ? For example,
Response rates provide a first-level indicator of advertisement effectiveness. Conversion rates offer a second-level indicator of advertisement effectiveness.
As comments have indicated, this is a specialised use and "first-level", "second-level" etc mean different things in different contexts. In this particular case, second-level is a better indicator than first-level.
If an advert is not effective, no-one will even respond to get more details of the product, let alone buy it.
Counting the number of responses to the advert will provide a measure of effectiveness. Counting the number of conversions — the number of actual purchases — will provide a better measure of effectiveness.
A more effective advertisement will have a higher number of conversions than a less effective one, although both might have around the same number of enquiry responses.
Because the quotation is ambiguous, I would phrase it
Perhaps you could think it this way:
first-level = top-level
second-level = sub-level
However, without the context, it's not easy to give a very definitive explanation here.
My guess: first-level indicator perhaps is a main/major indicator while second-level refers to subordinate/minor indicator.