In advertising baseline means brand signature. It is a short expression that completes the brand’s name or logo in advertisement.
This expression should be easy to memorize and it can be re-used on different media (TV, radio, …) . It is designed to last over time, contrasting with slogans which are texts used for punctual advertisement.
To be effective, a baseline should be effective, short, immediately comprehensible and reflect the image of the company and its products.