Selectivity Index, it's a measure of a particular substances effect on another in a specific set of circumstances. In this case they were saying that CQ (N4-(7-Chloro-4-quinolinyl)-N1,N1-diethyl-1,4-pentanediamine) (used to treat malaria and ambeasis) had a better effect at attacking the virus than Hydroxychloroquine (HCQ) sulfate, a derivative of CQ, was first synthesized in 1946 by introducing a hydroxyl group into CQ and was demonstrated to be much less (~40%) toxic than CQ in animals. That was indicative that CQ (although more toxic to the person) was more efficacious that it's realative, so now you need to measure the trade offs in using it over the other.

They seemed to be referring to different SI values based on different criteria, but overall they said:
"We need to point out, although HCQ is less toxic than CQ, prolonged and overdose usage can still cause poisoning. And the relatively low SI of HCQ requires careful designing and conducting of clinical trials to achieve efficient and safe control of the SARS-CoV-2 infection."