What do they mean with that RA overflow? I dont get it.
It's like... leftovers. Leftovers from a really good meal that'll help bring you to the next stage of existence.
e: relevant
The amount of beakers provided can be increased by an additive 25% of the median tech in two different ways: by controlling the Porcelain Tower, and by having the social policy Rationalism. These only matter at the time of resolution, and getting both of these boosts RAs to providing the full value of the median tech.
If the beakers granted by a research agreement exceed the cost of the technology being researched, overflow is applied to the next technology queued (via shift+clicking on the technology tree) or applied randomly if no technology is selected. Each research agreement resolving on a turn can complete at most one technology, along with an additional technology completed by standard research after the agreements resolve. Any overflow beyond completing these technologies is saved for the next turn.