The offer is based on a soft credit pull which does not affect your credit. However, you will still need to be officially approved before you can proceed. It is not an official approval but rather an invitation to apply for the actual product, no matter whether it is a loan or a line of credit.
After you apply, the company will make a hard pull on your credit in order to review your credit history in greater detail. Unlike a pre-approval, this hard pull will affect your credit score. But, not all hard credit pulls are the same, as it depends on the quality of your FICO score. But, on average, a hard pull will drop your FICO score by less than five points. According to FICO, if your credit history is strong -- with no other credit issues -- you'll see less of a drop in your score than someone who has issues with their credit. Fortunately, the drop lasts a few month, provided that other facets of your credit history are in good standing.