What are RTD-b positions in Italy?
Exactly, they are tenure-track assistant positions. After 3 years, if you pass a successful evaluation based on your research activity (abilitazione scientifica nazionale), you become an associate professor. The guidelines for this evaluation include an indicative target number of papers and citations, which is field-dependent. This is only indicative, there is a national committee in charge of the evaluation, and they are supposed to always check case-by-case. 

The teaching load is quite low: 60-80 hours of frontal teaching per year, plus exams (warning: there is a lot of exams in the Italian system). As far as I know there aren't other obligations, apart from maintaining a good research output to get the abilitazione. Supervising PhD students is not required.

(The teaching load becomes 120 hours after you become an associate professor.)

The system for the final evaluation is still provisional and has to undergo a new reform in the next months. These positions were introduced in 2010, and things are still new and bleeding-edge. But in any case they are worth pursuing, there are not many other chances to get a permanent associate professor position in Italy.

To be eligible, you need at least 3 years of post-doctoral experience (comparable to an Italian "rtd-a" position).