As compared to the Free API, this Pro API provides a paid tier with significantly higher rate limits: 43,200 requests per day for the pro tier, and 17,280 per day for the basic tier. These rate limit tiers reflect Twitter Search API's user-authenticated and app-authenticated ratelimits, respectively.
Users of the basic tier do not incur a monthly fee if they stay below the daily quota; both tiers enable users to exceed their daily quota for $1/1k requests. Users are free to switch between the plans at will without having to change the endpoint in their code.
Besides significantly higher ratelimits, the pro API does not currently offer any additional functionality as compared to the free API, but that may change in the future.